Gemstones and the Artist’s Palette – Natural Mineral Colouring of Paint Pigments

Michelle Clark
BSc (Hons I) PhD FGAA

Introduction

To most gemmologists, the idea of grinding up a beautifully coloured gemstone would be something tantamount to sacrilege. The appreciation of many gemstones starts with their colour but extends to their apparent beauty, rarity and durability, the very definition of what constitutes a gemstone. But this relatively modern, applied definition catapults what are essentially minerals, and occasionally biogenic materials, into other-worldly objects of desire, personal adornment and tradeable commodities. Relatively recent intervention of human skill to add value by the sophisticated cutting and polishing of minerals and other materials has turned them into objects with a perceived or “real imagined value” (Raden, 2015, p.2).

Yet coloured minerals are valued for many reasons. From antiquity until the advent of the scientific revolution, particularly during the 16th and 17th Centuries, colour in the ancient world often had a divine significance and was central to belief systems, cultural rituals and even treatment of disease. The interest in minerals stretches back millennia because they are, almost uniquely among natural materials, able to keep their colour in contrast to the transient colour of biological materials derived from plants and animals (Fritsch and Rossman, 1987). With little manipulation, natural sources of colour, such as the iron oxides found in the earth and rocks in much of Australia, are used as the source of pigment for painting and personal decoration, and are still visible in ancient cave art. But it might come as a surprise to many gemmologists to know that there are many gemstones that are used to create natural pigments which “may offer a renaissance in our understanding of color language” and have “unique properties [to] offer to contemporary painters” (O’Hanlon, 2013a).

The importance of colour

It was the work of Isaac Newton in 1672 on the refraction of light through a prism that demonstrated colour is an intrinsic quality of light and does not arise simply from light passing through a medium (Fara, 2015). Colour is dependent on the source of the light and the way in which it interacts with an object. Light is transmitted, reflected or absorbed and the combination of these possibilities produces the colour in the defined part of the electromagnetic spectrum (400nm-700nm) that the human eye can perceive. The relative absorption of longer (red) wavelengths of light by, for example, lapis lazuli, and the concomitant reflection of shorter (blue-violet) wavelengths, results in the gem appearing blue (O’Donohue, 2006; O’Hanlon, 2013b).

Theories of colour are complex – they involve the excitation of electrons and are interpreted by the physics of quantum theory and excitation and vibration energy levels, or in molecular orbital theory and optics theory. Transition metals, such as iron, chromium, copper, nickel, cobalt and manganese, are common in many gem materials as trace components or as part of the chemical compound and produce coloured salts. In addition to the chemistry and physics of colour, colour is intimately linked to biology: how each of us perceives or registers colour is due to the number and sensitivity of the photoreceptors and light-detecting cells (rods and cones) of the eye. Compounding variations in the biology and physiology of colour perception, differences in psychology such as colour preferences, terminology, symbolism and cultural determinants, affect individual interpretation of, and meaning ascribed to, colour (Nassau, 2020).

The components of colour

There are three components of colour:

• hue – the colour itself or wavelength composition eg red;
• brightness – the lightness or darkness of tone, also called value, which relates to the amount of light absorbed;
• saturation – the intensity or richness of colour which is a measure of purity or the degree to which it deviates from white or black (Hughes and Hughes, 2020).

In the world of gemmology, each of these components yearns for description and is inherently important in the determination of one of the described values of a gem. Precise description of colour is difficult, influenced as it is by the visual acuity of the observer, light source, pleochroic nature of the material, ability of the material to fluoresce, even the presence of surrounding colours and historical arbitrariness (Figure 1). But whilst this challenge remains, remove the trade constructs of cut, clarity and carat weight, which are relevant only when intensely coloured and vibrant minerals are manufactured into jewels, the constant and immutable feature of these minerals is their colour.

The artist’s desire for natural mineral pigment

Just as it is to those interested in gemstones, the concept of colour is paramount to artists. The manipulation or representation of colour has been an ongoing quest throughout art history. The imitation of natural objects, the re-interpretation of light and shade, the impression of reflection of light from surfaces, have all led to the need, and the search for, sources of colour that are durable, different and desirable. This search continues today. 

There is an ongoing quest to create new colours or shades that are deemed to be ‘missing’ from the artist’s palette (Coles, 2018). The fundamental desire is to create an enduring work that is greater than the sum of the parts, to satisfy the basic need to excite emotion, and tell a visual story which can be shared. To this end, the use of natural pigments has always been part of the artist’s repertoire (O’Hanlon, 2013a).

Sources of natural colour pigments include bone, either natural or burnt, to create charcoal; animals such as the insects from which cochineal is extracted; plants that are used to create indigo and woad; and minerals such as malachite, azurite and lapis lazuli, all of which are prized for their distinctive shades of green and blue; and ochres typically coloured by iron compounds. The creation of enduring pigments from natural minerals has crossed many time zones and cultures, and natural pigments continue to be important today, Centuries after the advent of synthetic dyes (the first one was Prussian Blue, discovered by accident in the early 1700s; Paints and Pigments, n.d.). To an artist, unlike a gemmologist, the value of a material is in the colour of a mineral that can be extracted, mixed, stabilised and then applied to a surface in order to tell a story. But, as it is in the world of gemmology, colour and its relationship with light, are the name of the game.

Developments in chemistry over the past 300 years have provided artists with thousands of synthetic colours from which to choose.
According to O’Hanlon (2013a), the overall “chroma” of a pigment – which he defines as the combination of hue, saturation and tone – is reliant on the particle size and shape of its chemical constituents, which, in turn, modify the degree of light absorption and reflection. The particle size of synthetic pigments is small (1µm) and uniform, such as cobalt blue created by the mixing of cobalt chloride and aluminium chloride. This offers technical advantages of product consistency and even suspension of the particles in the paint carrier.

Larger and less uniform particles (1-120µm) result from the grinding of natural minerals such as the secondary copper carbonate, known as azurite [Cu₃(CO₃)2(OH)₂], as well as the intergrowth of natural minerals, in this case, commonly malachite [Cu₂(CO₃)(OH)₂]. The heterogeneity of the particle size means that the resulting pigment reflects and transmits light in different areas of the visible spectrum compared to the synthetic pigment. Refraction of light by multiple layers of paint pigment by the varying particle sizes is coupled with increased reflection off the surrounding particles of paint binder. The result for the artist is greater luminosity and more vibrant colour (Gazo, 2020; O’Hanlon, 2013a; Figure 3). At a personal and emotional level, the subtlety of the colour also creates a direct connection between the minerals from which the pigment is extracted and, for some, the earth about which the artist paints (Broadhurst, 2020, pers. comm.; Figure 4).

Blues and Greens

Lapis lazuli

To the ancient Egyptians the colour blue, being the colour of the sky, was associated with creation and divinity. It was highly valued and thought to bring prosperity and protection. Lapis lazuli, turquoise, blue paste (glass) and Egyptian faience (a ceramic material) are commonly seen in ancient artifacts and items of jewellery (Fletcher-Jones, 2020; St Clair, 2016); fine colour lapis lazuli and turquoise are still prized in contemporary jewellery.

Very few blue pigments are available in the natural world. The colour ‘Egyptian Blue’ was first used around 2500BC and later described by the Romans; the Egyptians referred to the pigment as iryt (artificial) hsbd (lapis lazuli; St Clair, 2016) and it was made following precise chemical and temperature conditions. Whilst the original ‘Egyptian Blue’ contained a copper mineral such as malachite (discussed further below) and was an early development in the ceramic glaze process (Coles, 2018), lapis lazuli (from Latin, ‘the blue stone’) has been documented from as early as 630AD to create pigment which is still known as ‘ultramarine’. Then, as now, the main source of this well-known gemstone was Afghanistan (Emerson, 2015; St Clair, 2016).

Chunks of lapis lazuli were traded along the Silk Road, the ancient trading route along which goods were carried between the East and West. Gemmologists prize lapis lazuli as a deep blue, heavily pigmented ornamental gemstone (Figure 5a) that has been used in jewellery and ornamentation for millennia (Figure 5b). Lapis lazuli is a rock, being a mixture of minerals including lazurite (the sulphide-rich variety of hauynite), nosean, calcite and pyrite (responsible for the flecks of gold). Ultramarine blue pigment made from lapis lazuli is stable and is one of a few natural pigments with a vivid blue colour and good opacity (Natural Pigments Inc., 2020).

Lapis lazuli-coloured pigment has always been expensive and frequently a representation of wealth, power and devotion. In decorative objects and items of jewellery, lapis lazuli has always held the same significance (Emerson, 2015; Hartwig, 2020). In the manufacture of pigment, the rock is difficult to grind and because of the variability of the nature of its constituents, it needs to be purified by a complex and laborious process which has its origins in 9th Century alchemy (Coles, 2018). Ground lapis lazuli is mixed with gum mastic, rosin which acts as an emulsifier, beeswax, and linseed oil; the resulting mixture is heated to form a paste which is then kneaded with an alkali to form a dough (St Clair, 2016; Coles, 2018; Cennini, 2019; Broadhurst, 2020, pers. comm.). This treatment is designed to extract the pure lazurite which comprises 25% to 40% of the mineral composition of the lapis lazuli rock. Lazurite is a complex feldspathoid silicate sulphate [(Na,Ca)₈(AlSiO₄)₆(SO₄,S,Cl)₂], with the presence of sulphur responsible for the intense blue colour. The result of the time-consuming and difficult process is a pure blue comprised of around 80% lazurite, unimpeded by tinges of other hues such as green or violet, and undiluted by grey (Figure 6). According to Cennini (2019), “Ultramarine blue is a color illustrious, beautiful, and most perfect, beyond all colors” (p36). The use of ultramarine reached a peak during the Renaissance (Figure 7) and was used in many iconographic paintings and depictions of hallowed religious figures, particularly the Virgin Mary, until the synthesis of ultramarine pigment in the 1820’s (Emerson, 2015; St Clair, 2016). This new synthetic was readily available, reliable and much less expensive than the naturally-derived pigment.

However, according to artists at the time, and indeed many contemporary artists, the use of natural lapis lazuli in the preparation of pigment is preferred. St Clair (2016) writes “Artists complained that it [synthetic ultramarine] was too one-dimensional. Because the particles were all the same size and reflected light in the same way, it lacked depth, variety and visual interest of the real thing” (p.186). The laboriously extracted natural pigment is still used by purists and those artists seeking a special adherence to natural materials, a connection with the landscape that they are painting, or by those who appreciate a depth of texture not available with modern synthetic pigments. Western Australian artist Sam Broadhurst, after completing the difficult process to extract the blue pigment from lapis lazuli rock (with a recovery rate of just 3%), justifies the use of lapis lazuli in his work: “The most obvious use for lapis lazuli is the sky and water within the landscape, especially coastal scenes. Another overlooked application is with the blue greys of cliffs, and other areas such as sand shadows” (Broadhurst, 2019; Figure 8).

Azurite

Azure blue is the colour of the sky on a clear day, at least in the Northern Hemisphere. David Coles is a British master pigment-maker who emigrated to Australia and “attempts to replicate the light-filled blue of the Australian sky, which is vastly different to that of Europe or America” (Coles, 2018; p.xi). But for millennia, and for most art purposes, an azure blue colour was achieved by using another intensely-coloured natural gem mineral, azurite (Figure 9).

Azurite is a secondary copper carbonate mineral [Cu₃(CO₃)₂(OH)₂] known for its vibrant and, well, azure blue colour. Crystallising in the monoclinic system and with a low Mohs hardness typical of carbonates of 3½-4, azurite is frequently altered to malachite and the two minerals often occur together as the gemstone known, unsurprisingly, as azurite-malachite (Figure 2). Unlike ultramarine, pigment derived from azurite has a green undertone (Natural Pigments Inc., 2020) and was known in times past as ‘citramarino’, meaning “a blue from this side of the sea” (Coles, 2018, p.47). Hence, artists use “ultramarine to give height to the skies, and azurite to give depth to the seas” (Finlay, 2004, p.287). Not as expensive as lapis lazuli, pigment derived from azurite was often substituted for lapis lazuli and was used from the Middle Ages (5th Century to around 14th Century) through the Renaissance (14th to 16th Centuries) in Europe. Azurite pigment has also been found in wall paintings in Central China from the Sung and Ming Dynasties (approximately 960 – 1644 CE), as well as at the collection of Buddhist temples at Mogao caves in Dunhuang in Western China, painted from the 4th to the 14th Centuries (Douma, 2008; Kogou et al., 2020).

The process of pigment extraction is as laborious for azurite as it is for lapis lazuli but the resulting powder is not as stable and more coats are required to achieve opacity (Coles, 2018). The process of extracting the colour, by extensive washing and decantation of the resulting ground powder, has not changed since the method documented by Ambrogio di Ser Pietro da Siena in his Ricepte daffare piu colori in 1462, the first steps of which glow with reverence and have been translated as “When you want to refine azurite (l’acurro de la Magna), take three ounces of honey, as light as you can get, and cut it with a little hot lye, not too strong” (Cennini, 2019, p.35).

Cobalt blue spinel and glass

“Michelangelo would have liked this blue best. It is expensive, and leans towards violet” (Finlay, 2004; pp296-297). Used widely by the inhabitants of ancient Persia (now Iran) on the roofs of mosques to represent the heavens, and known since the 1500s, ‘cobalt blue’ was also coveted by the Chinese for hundreds of years. Widely available as a stable and intensely coloured pigment from the 18th Century it was used extensively by the Impressionists and Post-Impressionists. Created as a substitute for ultramarine (St Clair, 2016), cobalt blue is derived from the ore smaltite which is a toxic mixture of cobalt and nickel arsenide minerals. Cobalt has been used in the colouring of glass since Egyptian times and is known as ‘smalt blue’ when melted with lime silica and potash (Douma, 2008). The pigment is gritty and difficult to work with but for a time was a cheap alternative to ultramarine and azurite (Coles, 2018).

Natural cobalt-bearing spinels are very rare and are coveted as gemstones due to their vivid, high-purity blue hue (D’Ippolito et al., 2012; Figure 10). Synthetic cobalt blue spinel (CoAl₂O₄¹) is produced by mixing of cobalt oxide and aluminium oxide in varied ratios at high temperature to form a crystalline material (Natural Pigments Inc., 2020). This synthetic gem is well-known by gemmologists as manufactured by the Verneuil flame fusion melt process. The presence of the cobalt chromophore creates a characteristic spectrum of three broad bands in the visible spectrum at 635nm (orange), 580nm (yellow-green), and 543 nm (green), a strong red reaction under the Chelsea Colour Filter and red UV fluorescence. Due to the strain imparted on the cubic crystal lattice by the unnaturally high alumina content, Verneuil synthetic spinels show diagnostic extinction patterns when viewed through a polariscope (GAA, 2021).

Cobalt blue pigments have good ultraviolet and visible light opacity, are chemically inert, heat resistant, non-bleeding and have exceptional durability and opacity. The intense, blue-coloured alumina material can be modified with the addition of other metal oxides: cobalt green contains zinc oxide; cerulean blue contains tin oxide; and a greener shade of blue is created by the addition of chromium oxide (Douma, 2008; Natural Pigments Inc., 2020). The colouring of materials, typically natural gemstone imitations such as glass and spinel with cobalt solutions, continues today (Fischer, 1990). Specifics of the synthetic methodologies aside, whether the material is destined to become a gemstone or a paint pigment is a matter of how big it is – large pieces that can be cut and faceted are gemstones; when smashed into pieces and sieved, the particles create blue pigments of various intensities and are used for artistic purposes (Coles, 2018).

Vivianite

Also known as blue ochre, vivianite [Fe₃(PO₄).8H₂O] is a rarely encountered mineral in the gem world but coveted by collectors. Crystallising in the monoclinic system, it is an extremely soft (Mohs hardness 1½-2) iron phosphate mineral with strong pleochroism and unusual vitreous to mother-of-pearl lustre (Schumann, 1999; O’Donohue, 2006). Vivianite is a secondary mineral found in ore veins as an alteration of primary phosphates and found as sedimentary concretions such as in peat bogs (O’Donohue, 2006; Natural Pigments Inc., 2020).

Iron is a common chromophore found in many transparent gemstones often creating blue, green and yellow hues, notably in fine Australian sapphires, and whose pigmenting properties were the first to be recognized among the transition metals. In shades of green and blue, due to oxidation of the colourless mineral, vivianite as a blue pigment was known only from 12th Century paintings onwards and widely used as an alternative to the more expensive lapis lazuli in medieval paintings, particularly iconography from Europe. It was a relatively stable pigment although yellowing has resulted in a greenish overtone in some 17th Century Dutch paintings. Amongst afficionados of the art world, there is evidence of increasing use of it in recent years (Broadhurst, 2020, pers. comm; Gazo, 2020).

Malachite

To the gemmologist’s eye, there are few ornamental gemstones that are as instantly recognisable as malachite (Figure 11). Saturated and distinctively coloured in varied shades of green to nearly black, the concentric banding (which can appear as straight lines, depending on how the rough material has been cut) is a result of the natural botryoidal growth of its fine, needle-like crystals. The distinctive colouring and pattern, and with a Mohs hardness of 3½-4, make malachite a continued favoured gemstone for the creation of objects, for carving, and for use in contemporary high-end jewellery (Fetherston et al., 2017; Figure 12).

Malachite, like azurite, is a basic copper carbonate mineral [Cu₂CO₃(OH)₂] and also crystallises in the monoclinic system. It is the copper that gives both the mineral, and the pigment derived from it, its intense green colour, which is known in the artist’s world as “verde azzurro”, literally “blue-green” (Finlay, 2004, p.266; Cennini, 2019, p.31). Locally, malachite is sometimes found with chrysocolla (see below) and azurite in many copper deposits in Western Australia (Fetherston et al., 2013) and it is from this location that artist Sam Broadhurst collects the minerals from which he creates his pigments (Broadhurst, 2020).

Until the Renaissance, there was a taboo on mixing different substances – due to the inherent distrust of alchemists – so that combining blue and yellow was not done and green was difficult to obtain (St Clair, 2015). This lack of stable green pigment continued until the 19th Century when the copper-containing pigments were rediscovered, one of the first non-toxic green pigments being obtained from malachite. The copper-bearing mineral was used extensively by the Egyptians as a pigment as well as a cosmetic, believing that it might shield the eyes from the sun. Malachite was also used in China during the 8th Century to colour depictions of Buddha’s haloes and has been used in art through other parts of Asia for hundreds of years (Finlay, 2004). Malachite was further used throughout the Renaissance to illuminate mediaeval manuscripts and significant artworks (Coles, 2018). Intensely-coloured pigment is extracted by grinding selected material: the coarser the grind, the brighter the green. “For the sake of the color, work it up very, very little, with a light touch; for if you were to grind it too much, it would come out a dingy and ashy color” (Cennini, 2019, p.31). Washing and standing for “an hour, or two or three” and repeating this process results in a colour that “will be still more beautiful” (Cennini, 2019, p.32). Modern malachite-based pigment can be obtained in different tones, light and dark, based on the fine (20 µm; Figure 13) or medium (6-100µm) grind, respectively. The result is a more subtle, luminous glow than synthetic pigments (Natural Pigments, 2020).

Chrysocolla

Chrysocolla is also a secondary copper mineral of variable composition with a range of hardness (Mohs 2-4), variable refractive index, specific gravity and translucency. It is a hydrous silicate [(Cu,Al)₂H₂Si₂O₅(OH)₄.nH₂O] in the orthorhombic crystal system, and with a nod to the diverse minerals found across Australia, is found widely in Western Australia in copper ores with its associated copper-containing minerals malachite, azurite and turquoise (Fetherston et al., 2013; Broadhurst, 2020, pers comm.). Botryoidal or microcrystalline (ornamental varieties), and tending to a turquoise-blue, other colours can also occur generally in the range of bluish-green, bright green or light blue (Figures 14 and 15).

Used in ancient times, the name chrysocolla was first used by Theophrastus in 316BC – from the Greek “chrysos” (gold) and “kolla” (glue), in reference to it being used as a solder for gold (Coles, 2018; Mindat, 2020a). As a pigment, it is “delicate, pale blue-green” and in “oil painting it is very translucent, due to the pigment’s low tinting strength” (Coles, 2018, p.53). As with its fellow copper-bearing minerals, chrysocolla can be prepared by successive rounds of grinding from a coarse to a fine powder which, when handled skilfully, renders a wide range of colours. Artist’s pigment from chrysocolla, commonly referred to as ‘Cedar Green’, has also been used since antiquity and has been found in twelfth dynasty (approximately 1985-1795BC) Egyptian tombs, and possibly used as a watercolour paint during the 16th and 17th Centuries (Gazo, 2020).

Dioptase

A somewhat bluer green than malachite but found in the same locations in copper ore deposits, dioptase [CuSiO₂(OH)₂] is a soft mineral found on calcite and “makes a most aesthetic mineral specimen, but its small rhombohedral crystals are commonly heavy” (GAA, 2021; O’Donohue, 2006). The name dioptase dates to 1797 when the French mineralogist, René-Just Haüy, one of the founders of modern crystallography, considered the visibility of the cleavage planes (perfect in 3 directions) through the transparent faces of the crystal, deriving the name from the Greek “dia” (through) and “optos” (visible; Mindat, 2020b). Faceted dioptase is rare (the Mohs hardness of 5 doesn’t help) but crystallising in the trigonal system, the strong and bright blue-green coloured rhombohedral crystals have been used to stunning effect in one-off jewellery (Figure 16).

Thought to be used since ancient times, dioptase as a natural pigment has not been in widespread use for over a decade, in part due to lack of availability. The majority of specimens from which the material is used for gems and jewellery come from the Altyn-Tube region of Kazakhstan but also from Namibia, USA (Arizona) and Zaire’s Congo (O’Donohue, 2006; Natural Pigments Inc., 2020). The fine pigment that is extracted from the crushed mineral is a vibrant, vivid green which is similar to, but more intense than, chrysocolla (Gazo, 2020; Figure 17).

Red, Orange, Yellow, Black, White and More

Crocoite

Crocoite (PbCrO₂) occurs as a distinctive, transparent-translucent crystal in the monoclinic system, typically with an acicular habit, bright, adamantine lustre and vibrant orange colour (Figure 18). Of limited use as a gem due to its softness (Mohs hardness 2½-3), brittleness and good cleavage in 3 directions, crocoite is nevertheless a sought-after collector’s mineral.

Rarely are crystals large and transparent enough to facet and pose such a challenge for lapidarists that faceted material generally makes it only as museum specimens (Figure 19; R. Pogson, Scientific Officer Collection Manager, Mineralogy & Petrology, Australian Museum, 2020, pers. comm.). Found in several geographic locations as diverse as Tasmania and the Ural Mountains in Russia, it is the latter that produces the only source of sizeable and quality facetable material. Crocoite is found in oxidised zones of lead deposits with chromium-bearing rocks. The mineral formerly known as ‘crocoise’ (1832) is named after the Greek word “krokos” meaning saffron (O’Donohue, 2006).

Ground to a powder to extract the pigment as lead chromate, crocoite was described in 1797 by Louis Nicolas Vauquelin and found to contain what is now known as chromium (from the Greek, “khrôma”, colour). The resulting pigment was a triumph – intense and ranging in colour from golden yellow to rich orange or red (Coles, 2018). Artists in the 1800s – including Turner, Manet, Cézanne, Monet, Pissarro and Van Gogh – used this new pigment known as “chrome yellow” to create drama and combined it with other primary colours to emulate plein air light effects (O’Hanlon, 2013c; St Clair, 2016). Unfortunately, the colour fastness of the pigment was a problem and the yellow colour darkened over time. Changes made to the formulation in the 20th Century enabled the chrome pigments to be used again, and, depending on the particle size, light and dark shades are produced (O’Hanlon, 2013c).

Cinnabar

Cinnabar (HgS) is the only important ore of mercury, forming rare cochineal-red, translucent to opaque, rhombohedral form crystals in the trigonal system. It is soft (Mohs hardness 2-2½), has the highest RI of any mineral (ω ray 2.905 and Ɛ ray 3.256; O’Donohue, 2006), a high SG (8.09), shows perfect cleavage in one direction, and has a birefringence of 0.351 (O’Donohue, 2006; International Gem Society, 2020). Topped off with an adamantine to submetallic lustre, cinnabar is a fascinating mineral.

Cinnabar is fashioned as carved objects and due to its softness, is cut as cabochons. It is purported to be named after the Persian word “zinjifrah” meaning dragon’s blood, and the tale told by Pliny that it is “the result of an epic struggle by an elephant and a dragon…The merging of their blood created cinnabar” (Finlay, 2004, p.163). Cinnabar has a unique combination of mineralogical attributes, history and myth, with toxicity added for good measure.

The use of cinnabar as an historical pigment is known from the 3rdCentury BC in China, particularly in the colouring of red lacquer (Figure 20). It is still mined in China as well as in Italy, Turkey, Greece and Spain, the latter being the location of large resources of cinnabar known since Roman times where it was also used as a source of mercury (Finlay, 2004). Cinnabar is found in veins around hot springs and in low-temperature ore deposits (O’Donohue, 2006); the ground powder is a bright red pigment known as vermilion or ‘Chinese Red’ (Web Exhibits, 2020). There is still doubt as to the permanence of the pigment although the natural mineral is considered more stable that its synthetic counterpart. Whilst dry pigment is available for artists to purchase, it generally comes with safety warnings: although mercury in this form is the least toxic of all forms of the metal, it is still potentially dangerous to use (Crampton, 2020; Natural Pigments Inc., 2020).

Ochres, metal oxides and hydroxides

In mineralogical terminology, ‘iron oxides’ may refer to the two main iron oxide minerals: hematite and magnetite. For the gemmologist, hematite (Fe₂O₃) with its attractive metallic lustre, is often found as inexpensive beads (which may or may not be magnetic) or carved as intaglios (Figure 21). Magnetite (Fe₃O₄) is a dense, brownish-black to black, highly magnetic mineral and an ore of iron. Like hematite, the mineral is opaque with a metallic lustre; it is crushed and milled to a fine pigment to create a strong, transparent black used in many painting media (Gazo, 2020). 

Easy to find, plentiful and stable, the natural clay earth pigments of metal oxides and hydroxides are widely termed ochres (Figure 22). In the art world, hematite, which has a red streak, is known as red ochre; limonite [FeO(OH)nH₂O] is known as yellow ochre; and goethite [α-Fe³⁺O(OH)] is known as brown ochre. They have all been used as sources of pigment for millennia.

Ochre-based pigments were the first colour paint and have been used by all indigenous peoples on every continent (Finlay, 2004; St Clair, 2016). Australia has the “longest continuous painting tradition in the world” (Finlay 2004, p.27) with the oldest known cave painting dated to almost 30,000 BCE. The use of ochre-derived pigments are still culturally important in traditional Aboriginal life, art and ceremony (Finlay, 2004; Artlandish, 2020). To the contemporary artist, iron oxides from the Pilbara region of WA provide “a wonderful source of earth colours” (Broadhurst, 2020, pers. comm.).

Ilmenite (Fe²⁺TiO₃) is a black mineral rich in iron as well as titanium and was first used as an art pigment in the 1920s after a method was developed to remove the iron and create a white pigment. Treatment with sulphuric acid then hydrolysis results in a white precipitate of hydrated titanium dioxide (TiO₂). Current processes are different but the result is a safe (unlike early lead-based white pigments that were used for hundreds of years prior) and exceptionally opaque white that is used in many applications including paint, plastics and printing inks; according to Coles (2018) “It is the most widely used pigment of all time” (p.145). Rutile, derived from ilmenite sands and which also contains a small percentage of iron, is used as the base for buff-coloured pigment: “The small percentage of iron oxide remaining gives buff titanium its characteristic natural or “buff” color… ground to a specified fineness with particles slightly larger and more irregularly-shaped than typical titanium dioxide white pigment” (Natural Pigments Inc., 2020).

For the gemmologist, rutile is often encountered as interesting and attractive inclusions in gemstones, typically quartz, which can make such specimens coveted by collectors (Figure 23).

As modern, natural art pigments, iron oxides and hydroxides are known by a number of exotic names depending on the hue or shade: Indian red, English red, Venetian red, Sartorius Red, Mars red. Synthetic pigments in these colours are referred to in the art world as “Mars colours” (Natural Pigments Inc., 2020). Since the industrial revolution and the understanding of inorganic chemistry, modern pure oxides are the result of hydrolysis reactions of iron salt solutions. Colour and tinting strength are, as with other pigments, dependent on the concentration of water, particle size and the presence of other metals such as manganese, and natural ochres are more translucent than synthetics (Coles, 2018).

Manganese is a strong and varied contributor to the colour in many natural gemstones from the rosy pinks of common opaque ornamentals rhodonite and rhodochrosite; the pale pink to orange-pink of beryl morganite (Figure 24); the bright orange and the highly desirable reddish-orange (known as ‘aurora red’) of spessartine (Figure 25), or the trace amounts in hessonite (grossular) garnet; to the striking violet-purple of sugilite (O’Donohue, 2006; GAA, 2021; Figure 26). Pure manganese oxide can be used as the base of brown or black pigments in umber and sienna pigments. But manganese is also the source of “manganese violet” which, in 1881, led to Impressionist artist Claude Monet declaring “I have finally discovered the true colour of the atmosphere, it’s violet…”
(Coles, 2018, p.137). Later, “Manganese blue [that] is a clear azure blue” (Coles, 2018, p.137) was commercially produced in the mid-1930s but concerns about environmental toxicity led to its discontinuation in the 1990s (Jackson’s Art Supplies, 2017).

Other black and white pigments from inorganic and organic gem materials

Organic and biological materials (also referred to as biogenic materials; Galopim de Carvalho, 2020) that might be considered gem materials are many and their history is long. The transformation of these same materials into pigment is also well known since antiquity with bone, chalk, shells and pearls variously crushed and burned for both black and white pigments (Finlay, 2003; Coles, 2018). Ivory is rarely used nowadays due to changes in international laws such as CITES which is a multilateral treaty governing the trade in endangered plants and animals.

Cennini (2019) details in his “Il Libro dell’Arte” how to choose “What kind of bone is good for treating the panels” (p.5). Suffice it to say, the process is rather gruesome: “exposure to intense heat in the absence of oxygen turns the bones into carbonised char” (Coles, 2018, p.25) whilst the addition of oxygen turns the material to ash. The description of the full procedure will make non-carnivores cringe, and even rusted-on carnivores might recoil next time they see an oil painting and consider the “bodying qualities” of the white paint: “…ash is made from selected bones properly leached, ground, chemically treated, calcined by a special procedure and milled to a small particle size. …Add to oil colors and mediums to create textural and bodying qualities to oil paint without affecting the color.” (Natural Pigments Inc., 2020).

The iron-rich, commonly black variety of tourmaline known as schorl [NaFe₃Al₆(BO₃)₃Si₆0₁₈(OH)₄] (Figure 27), is also used for black pigment; marketing parlance poetically describes the versatility of this pigment “as dark as night or as pale as a wispy fog” (Smith, 2020). Australian artist Sam Broadhurst has attempted to create black pigment from schorl collected from an area north of Newman in Western Australia but admits that due to the difficulty of obtaining fine-grained material “charcoal was a much easier source of black pigment” (Broadhurst, 2020, pers. comm.).

The soft, fine-grained, easily pulverized, white-to-grey variety of limestone, chalk (geologically known as calcite), is a calcium carbonate (CaCO₃) also encountered in gemmology circles. In the form of optically pure calcite (Iceland spar), it is an essential component in the dichroscope for the determination of pleochroism due to its large double refraction (birefringence of 0.172; Figure 28). Calcium carbonate also occurs as the mineral aragonite (the less stable form which is generally responsible for the iridescence of mother-of-pearl; O’Donohue, 2006), marble, shell and pearl. Similarly, chalk appears in many guises in artwork with more romantic names based on the source such as English Whiting, Bologna chalk and Champagne chalk. Microscope examination of Old Master paintings often shows the presence of single-celled marine fossils called coccolithophores, the creatures from whom the white paint, in the form of chalk, was derived (St Clair, 2016). Think White Cliffs of Dover where large deposits of CaCO₃ in the form of chalk have created the stunning coastline on the south-east coast of the UK.

Widely used since at least Roman times as a component in gesso – a white plaster which is used as a base layer on many substrates – to thicken paint or prepare glazing layers, a use which is permissible due to a low refractive index (ω ray 1.658 and Ɛ ray 1.486) which makes chalk translucent in oil media. Both abundant and easily processed, chalk is ground, washed and sedimented into different layers of variable coarseness, viscosity and diaphaneity to give an ideal white pigment (Coles, 2018).

Special Effects

Mother-of-pearl and shell derived from various nacre-producing species of molluscs of the genus Pinctada, as well as New Zealand paua, Haliotis iris, are used directly in gilding techniques to give a decorative finish. Some shell is used in its natural state whilst some is dyed and laminated to create additional colour choices (Jackson’s Art Supplies, 2017). Naturally-occurring metal oxides such as titanium dioxide and the mineral mica are often added to synthetic pigments to give the resulting paint a pearly lustre and iridescence. These paints are reflective but, as is also the case with natural pearl nacre, the sheen thus created reflects light and allows the colour of the pearl (or paint) to be seen as well (Figure 29). The result is a “textured glow that is reflective without being overpowering” (Hager-Suart, 2019). Similarly, metallic paints can be created which also reflect the light but to a greater extent which generally masks the underlying pigment colour.

Conclusion

Wait, there’s more

Garnet, tiger’s eye, kyanite, purpurite, bloodstone, red coral, red jasper, spinel and amethyst. These minerals and materials are known and appreciated as gemstones but are also ground and further processed for a wide range of artist’s pigments (Jackson’s Art Supplies, 2017; Table 1). The creation of artist’s pigments from natural minerals for acrylics, oils, watercolours, pastels, inks and gouache is a chemical and artistic process, and the resulting pigments are endowed with “vibrancy, consistency and stability” (Hager-Suart, 2018). Mechanical grinding, mixing with the appropriate medium in the correct ratio is a craft and a passion that “involves the tenacious pursuit of the alchemical transformation of dirt into colour” (Coles, 2018, p.ix)².

The desire for stable and different colours has engaged artists for millennia. In the translation of Cennini’s “The Craftsman’s Handbook” (p.xvii), Thompson cites an example of rhetoric used by Cennini in the original text:

“[painting] calls for imagination, and skill of hand, in order to discover things not seen, hiding themselves under the shadow of natural objects, and to fix them with the hand, presenting to plain sight what does not naturally exist.”

This quote could just as appropriately be used to describe the art and science of fine lapidary work. Rough material as the “natural objects” are “fixed with the hand” by cutting, carving and faceting which unveils an object that is not otherwise appreciated in its natural state. Thus, natural pigments, much coveted by modern artists, are not dissimilar to the gemstones from which they are derived: durable, rare, beautiful, and desirable.

Acknowledgements

Thanks to Sam Broadhurst for generously sharing his knowledge and for allowing use of his images. Thank you to Léonie Rennie for reviewing an early draft of the manuscript and for her helpful and encouraging comments. Special thanks to Susan Stocklmayer for her considerable time reviewing many later versions of the manuscript, and for tweaking and coercing the final article into shape.

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Footnotes

¹ the generic formula of spinel is AB₂O₄ where A can be Co, Mg, Fe, Zn or Mn and B can be Al, Fe or Cr

² with apologies to mineralogists