“Lemon ‘n’ Lime” Chrysoprase: A Brief Overview of Chrysoprase from Marshall Pool, Western Australia

Murray Thompson
BSc CF FGAA

Acknowledgements

The author is grateful to Dr Peter Downes for his chemical analysis using scanning electron microscopy, and to Susan Stocklmayer for her gemmological analysis of the properties of the Marshall Pool chrysoprase. I am also grateful to Susan and Léonie Rennie for their constructive comments on earlier versions of this article.

This article begins with the origin and colour of chrysoprase, provides a little of its history, then describes the geological sources of the Marshall Pool variety. The chemical, gemmological and mineralogical properties of this chrysoprase are outlined, followed by a discussion of how the rough material can be worked into a range of shapes to exploit the “lemon ‘n’ lime” colouration and the green variety of chrysoprase.

Introduction

Chrysoprase is a gemstone variety of chalcedony (a microcrystalline form of silica) that contains small quantities of nickel. Its colour is normally yellowish- to bluish-green and can vary in intensity from light to deep green. Other ornamental varieties of the microcrystalline silica family include agate, carnelian and onyx.

As with all forms of chalcedony, chrysoprase has a Mohs hardness of 6–7 and a conchoidal fracture like flint. However, unlike many opaque silica minerals, it is the unique colour of chrysoprase, rather than any pattern of markings, that makes it a desirable gemstone for jewellery.

This article focuses on chrysoprase from Marshall Pool, Western Australia, noting particularly a variety that is an unusual combination of two materials: green chrysoprase and yellow-white magnesite. The resulting attractive colour has been informally named “lemon ‘n’ lime”. A carved, free-form specimen is shown in Figure 1.

The Origin and Colour of Chrysoprase

Chrysoprase results from the deep weathering or laterisation of nickeliferous serpentinites or other ultramafic rocks. In the Australian deposits, chrysoprase occurs as veins and nodules with brown goethite and other iron oxides in the magnesite-rich saprolite zone below an iron and silica cap. The colour of chrysoprase is due to trace amounts of nickel compounds, in the form of very small mineral inclusions, weathered from decomposing ultramafic rocks. The abundance of nickel determines the strength of the green colour (Jiang and Guo, 2021).

The best quality Australian chrysoprase does not show a grey overtone and is often described as a vivid ‘apple-green’. In general, it has been noted that the chrysoprase from the two main sources in Australia, Marlborough (Queensland) and Eastern Goldfields (Western Australia), is remarkably consistent in hue, saturation and tone. Another variety found in Western Australia is incorrectly called “lemon” chrysoprase. Rather, it is a pale opaque lime green colour and is not a chalcedony, but a nickeliferous magnesite. The boulder of chrysoprase shown in Figure 2 illustrates this mineral combination.

A Brief History of Chrysoprase

The word chrysoprase comes from the Greek χρυσός (chrysos), meaning gold, and πράσινον (prasinon), meaning greenish.

Chrysoprase is one of the historically early utilised semi-precious materials. In the crafting of seals, jewellery and other objects for their functional or aesthetic virtues, green and semi-transparent materials were common in the Hellenistic (323–30 BCE) and Roman periods. However, materials mentioned by Pliny (23–79 CE) as chrysoprasius also referenced plasma, prase, beryl and other green gemstones (Frondel, 1962).

Sources for the ancient use of chrysoprase are now lost but chrysoprase was quarried during the Middle Ages from Szklary, Lower Silesia in Poland; polished slabs are documented in the Chapel of St Wenceslas in Prague Cathedral in the Czech Republic, constructed at the end of the 14th Century (Čermáková et al., 2017). The use of the chrysoprase in this chapel is evidence that chrysoprase deposits in Europe were known of in the 14th Century. Of particular importance would have been the deposits at Szklary that were rediscovered in 1740, some of which was commissioned for use in the ornamental decoration of the Sans Souci Palace in Potsdam, near Berlin in Germany by the Prussian King, Frederick the Great. Readers interested in the archaeological and later history of chrysoprase will enjoy reading a recent article by Sachanbiński et al. (2023).

Geographical Sources of Chrysoprase

Whilst the deposits at Szklary were probably the largest occurrence of chrysoprase in Europe, there are deposits known in many countries throughout the world, including in Indonesia, Haneti in Tanzania, Germany, Russia, Arizona, California, Brazil, and Australia.

There are two major commercial sources of chrysoprase in Australia, each producing substantial quantities which comprise most of the material available on the market today. The first and earliest important source is the vast deposit found at Marlborough, Queensland in 1965. The second source was unearthed in Western Australia in the Yerilla District in 1992. There are many smaller deposits in the Eastern Goldfields and other places in Western Australia, and some of these are named with a location in Figure 3. The source discussed in detail in this article is the deposit at Marshall Pool. It is important to note, however, that whilst most chrysoprase is currently produced in Australia, it is not widely used because supplies are limited (this point will be returned to later).

Locality and Geology of the Marshall Pool Chrysoprase Deposit

Chrysoprase in Western Australia is found in the 600km-long Norseman-Wiluna Greenstone Belt, a region renowned for its gold and nickel deposits. The Marshall Pool area, located about 70km north of Leonora on the Goldfields Highway towards Leinster, is a particularly rich source of chrysoprase, and it has been described as a discrete part of the highly mineralised greenstone belt in the region of Mt Clifford (see Figure 3).

The northerly Agnew–Wiluna portion of this belt is highly attenuated and characterized by major strike slip (wrench) faults traceable over hundreds of kilometres, with at least two phases of complex folding. There is a general lack of rock exposure making mapping difficult.

Peak metamorphism expresses as lower greenschist facies in the region of Mount Clifford just 20km from Marshall Pool. The Mount Clifford-Marshall Pool block is characterized by gently folded sequences of mafic, ultramafic, and felsic volcanic rocks, most of which carry a substantial amount of nickel.

The Marshall Pool structure is estimated to be at least 15km long and rises approximately 10–20m above the plain. The company, Precious Gems of Western Australia Pty Ltd, has workings at the north end of the geological structure which the author visited in August–September, 2019. It is these newer workings that are the focus of this article.

Chrysoprase occurs either in magnesite/chalcedony nodules or as discrete nodules in the ferruginous caprock (Figure 4). Both the flat sheeted and the wider vertical veins are surrounded by carbonate alteration. The magnesite nodules and veins are commonly enveloped in a green siliceous material, informally termed “limecrase”, a mix of magnesite and chrysoprase, which, in turn, may be surrounded by chrysoprase.

Green colouration of the veins was noted at the original workings, within two metres of the surface and below a caprock of light brown magnesium carbonate crust, or “magcrete”. Darker green material was noted at depth, in thick veins and nodules, and in areas of more massive rock. The chrysoprase enveloping the magnesite nodules is commonly dark coloured, translucent and free of inclusions. Near the surface, the magnesite-rich segments of the nodules have been leached, leaving cavities, and the chalcedony adjacent to the cavities and vein edges are coloured white. Weathering has leached the green colouration from the chrysoprase at the surface.

Description of Gem Rough from the Marshall Pool Chrysoprase Deposit

The gem rough material from the Marshall Pool deposit that the author inspected and has subsequently worked, presents primarily in nodular form with sizes ranging from less than 100g to boulders in excess of 350kg (Figure 5). Larger boulders have been noted at the site, but it is difficult to lift them! Vein material has been almost non-existent in the most recent workings.

The nodules consist of a large portion of magnesite, both weathered and unweathered, iron oxide material, detrital carbonate infill, and partly weathered and poor to high quality chrysoprase (Figure 6). Pure, high-grade nodules are rare, the largest seen being 1.1kg. Almost all nodules have gem quality chrysoprase present that could be cut but of more interest to the gemmologist is the transparency and variety of colour of the material.

Why is the colour variable? Is it related to the proximity of the material to the surface, or is it due to the weathering in a shear-zone, or perhaps it is the presence of another type of chromophore? It has been recorded that chrysoprase fades when subjected to heating and/or sunlight (Fetherston, et al., 2017). Could this colour variation be due to dehydration? In the author’s experience, yes, it is. Cutters (including the author) in the workshop have commented on the change of colour in some pieces as they have been worked, over a period of just a few days. That observation is by no means a scientific analysis; however, working closely with a lot of similar material over long periods of time does lead to a familiarity and appreciation that is more than just a casual observation. Also, several cutters from separate locations have noted the increasing translucency of the material through the polishing stages from the coarse sanding, and this has a noticeable effect on the colour. The lightening of colour coupled with an increasing translucency sometimes results in a ‘glow’ in the appearance, almost as if there is an internal light source.

The pronounced desiccation of the chrysoprase surface into the underlying yellow magnesite is very noticeable on all nodules, but at present this author does not have an adequately researched explanation. These cracks are often infilled by an iron-rich carbonate compound, leached down from the regolith above over millennia.

Comparison of Chrysoprase from Marshall Pool with the Deposit in Marlborough, Queensland

The chrysoprase deposits at Marlborough, Queensland, are also hosted by steep-dipping shear zones within ultramafic rocks. Weathering and laterization have also removed some of the green colouration (Figure 7), but this can persist to depths of 10-40m (Rolfe, 1993).

Geologically, the deposit at Marshall Pool is similar to that at Marlborough, having the same host rock and structural control. The chrysoprase material at both deposits is also similar. However, compared to the mineralisation at Marlborough, the mineralisation at Marshall Pool has a higher density of chrysoprase-carbonate veining, and a shallower depletion of colour. It also has a flatter dipping structure (Rolfe, 1993). These features indicate good prospects for the Marshall Pool deposit, the extent of which is not yet clear.

Gemmological Properties and Mineralogical Examination of Marshall Pool Chrysoprase

In Figure 9, a polished specimen of chrysoprase in reflected light shows its uniform green colour. The same specimen remains fully bright lit in various positions by transmitted light and under crossed polarized filters (Figure 10); this response is characteristic of a polycrystalline material with no textural orderliness. Under transmitted light, there is patchiness of the green colour within the specimen, and a narrow band of lighter colour through the centre section (arrowed on Figure 10). Other areas of lighter colour are the relative thinner zones of the shaped piece. The specific gravity of the specimen shown in Figures 9 and 10 was obtained by hydrostatic weighing and is 2.63.

The refractive index from a polished surface gives a diffuse reading at ~1.55. Inspection of the absorption spectrum reveals no absorption bands but there are strong transmission windows of green, yellow and a small part of the orange/red and the spectrum is foreshortened at both the blue and red positions. Under a Chelsea colour filter (CCF), the specimen appears grey-green.

To gain more information about the mineral compositional information a sliver was removed from the chrysoprase and examined under a polarising microscope. The image in Figure 11 is a photomicrograph of the sliver and shows that the specimen consists of a mosaic of irregular quartz crystals of various mixed states.

Conclusions

From limited tests and observations, the mineral sliver shows that the chrysoprase has a predominantly polycrystalline quartz composition consisting of a mosaic of irregular shaped quartz of varying sizes. The gemmological results are consistent with this quartzose identification. The lack of absorption bands in the visible spectrum and absence of pink-red colouration under CCF are indicative of the material having no trace amounts of chromium. Chromium is a natural cause of green colour in other varieties of chalcedonic quartz, such as that found in the Newman region of Western Australia (Fetherston et al., 2017)

Chrysoprase is not a single mineral but an assemblage of silica (SiO₂) minerals, and analyses of chrysoprase specimens from around the world “show very diverse microstructural forms [of silica], e.g. fibrous, radial-fibrous, spherulitic (globular), checkerboard-like, mosaic, rosette and mixed” (Sachanbiński, et al., 2023, p.5).

In summary, the chemical, mineralogical and gemmological results have established that the cabochons and mineral specimens tested would be termed chrysoprase, a microcrystalline quartz material coloured by the presence of nickel as the major chromophore

Working the Gem Rough

Buying parcels of translucent, gem-grade chrysoprase, either in the field or the workshop, is fraught with issues. As already alluded to at mine-scale level, there are changes within every nodule of the Marshall Pool material, whether they be fist-sized or weighing hundreds of kilograms. Figure 12 shows the washing of a large nodule to reveal the colour change within. Every novel geochemical combination of silica, nickel, magnesite, and oxide minerals is visited within this gem medium. It is, then, the gem cutters’ business to unpack these nuances, add them to their knowledge bank, reinterpret, resolve, and move on again… sometimes quite tentatively.

Rough material containing chrysoprase is tough to saw, grind and shape. Over decades of processing this material, the author has found that sawing the rough gives the best insight as to the quality of the specimen. Even so, the diamond blades on the slabbing saws need to be checked constantly for sharpness as they glaze up quickly. Glazing occurs when a diamond saw is cutting material too soft to wear away the bonding material that encases the diamond grit, or when a hard, fine-grained material, like chalcedony, is insufficiently coarse to wear the bonding and expose new diamonds. Saws are de-glazed by cutting into an abrasive material to expose new diamond cutting zones.

Interpretations of cutting in terms of the variation in mineralogical structure and colour

When cutting, for the author, it is geologist meeting lapidary appreciating the association between the ironstone matrix and chrysoprase, particularly when it has a little silicate crust added for good measure! Figure 13 (left) shows this silicate crust on the right edge of the specimen. The tri-coloured specimen (Figure 13, right) demonstrates the three common variably-coloured materials that make up this gem along the northern section of the Norseman-Wiluna Greenstone Belt. The tough, light green to yellow magnesite is the gem variety of a normally white weathered rock that is found in every open-cut nickel mine along the Norseman-Wiluna Belt. Light coloured chrysoprase is found and collected from the same belt, the colour provided by nickel leached from the surrounding host rock by surface-water over millennia. Lastly, ferricrete (a siliceous iron oxide, usually stratified and very tough to penetrate even with heavy mining equipment) is found throughout Western Australia. To get all three coloured materials together in a fine-grained, semi-translucent piece of gem rough that takes a very high polish in just a few centimetres is uncommon but is very rewarding to cut and shape.

Figure 14 shows material with the two colours, informally named “lemon ‘n’ lime”, a chrysoprase that seems to be a specialty of the Marshall Pool deposit, as the author has not seen it elsewhere in a worked form. The gradation of colour is a very attractive feature of this very particular chrysoprase; it is difficult to source even from the kilograms of potential material at Marshall Pool.

Another of the interesting colour combinations from Marshall Pool is illustrated in Figures 15 and 16. The magnesite portion of the nodules is white, as shown in the split nodule (Figure 15). The region of gradation between white and green is reminiscent of an ocean wave: white foam in a green sea and informally it could be described as “sea foam”. When cut appropriately, the very fine-grained magnesite intermixed with equally fine, translucent chrysoprase makes a spectacular combination (Figure 16).

While the author’s preference is to fashion all the various minerals around chrysoprase into a whole piece, sometimes the pure richness of translucency and colour of the green chrysoprase has to be celebrated for what it is. The high-domed cabochons are beautiful to work as they hold colour so well due to their depth, a feature quite evident when working through the various sanding and polishing stages. Figure 17 shows an experimental carving of some of the chrysoprase from Marshall Pool in another attempt to capitalise on the depth of colour; Figure 1 also shows a carved piece but which exploits a mixture of the “lemon ‘n’ lime” colours.

Availability of Chrysoprase Internationally

In Australia, particularly in Western Australia, the lapidary and mineral fraternity see a good deal of chrysoprase; at times almost swamped with it. However, globally, this is a very real and gross anomaly. Sachanbiński et al. (2023) remarked that the chrysoprase market has varied over the centuries, with the discovery of more sources matched by increasing demands from craft workshops; they consider the peaceful green colour of chrysoprase to be desirable, with green generally believed to have a positive effect on the human spirit. Sachanbiński et al. also list some of the well-known jewellery designers who have worked with chrysoprase and note that in current markets, chrysoprase is sought after because of the decorative value of its colour. High quality chrysoprase, particularly when pieces are well-crafted, commands high prices, as does antique jewellery featuring chrysoprase.

A comment from Dr Hobart M. King (a GIA graduate) appropriately sums up the current commercial position of chrysoprase:

“Although chrysoprase is a highly valued variety of chalcedony, it is rarely seen in the gem and jewellery market. Thus, it is out-of-mind with the average jewellery shopper. It is rarely seen in commercial jewellery because stones of consistent colour and calibrated sizes are difficult to obtain in large quantities. Chrysoprase is most often seen in stores and on websites that sell designer, limited production, or one-of-a-kind jewellery items”. (King, 2021)

Sometimes it is only by chance that the best rough materialises at all, and that it is perseverance by both the miner and the lapidary that pays off in the end to create a beautiful piece from Australian chrysoprase.

References

Čermáková, Z., Hradit, D., Bezdička, P. and Hradilová, J. 2017. New data on “kerolite-pimelite” series and the colouring agent of Szklary, Poland. Physics and Chemistry of Minerals, 44, pp.193-202. DOI: 10.1007/s00269-016-0848-z.

Fetherston, J., Stocklmayer, S. and Stocklmayer, V., 2017. Gemstones of Western Australia, 2nd ed. Perth, Geological Survey of Western Australia, Mineral Resources Bulletin 25.

Frondel, C., 1962. Dana’s The System of Mineralogy. Volume III, Silica Minerals. New York, John Wiley & Sons, Inc.

Jiang, Y., and Guo, Y., 2021. Genesis and influencing factors of the colour of chrysoprase. Scientific Reports, 11(1), 9939. DOI:10.1038/s41598-021-89406-x.

King H., 2021. Chrysoprase: the second most valuable variety of
chalcedony. Geoscience news and information. [online] Available at <https://geology.com/gemstones/chrysoprase/#:~:text=Chrysoprase%20is%20most%20often%20seen,like%20fine%20chrysoprase%20or%20jade> [Accessed 18 May 2021].

Rolfe, G., 1993. A geological evaluation of the Marshall Creek chrysoprase tenements, Leonora, Western Australia. Unpublished progress report WA Mines Department A40772.

Sachanbiński, M., Kuleba, M., and Nowak, N., 2023. Chrysoprase – history and present. Mineralogia, 54, pp.1-10. DOI: 10.2478/mipo-2023-0001.

Images courtesy of Desert Fire Designs unless otherwise stated.