THE AUSTRALIAN GEMMOLOGIST | A Story in a Gemstone Necklace
A Story in a Gemstone Necklace
Figure 1. The necklace showing the range of collet-set gemstones interspersed with gold chain.
Introduction
The multi gemstone necklace (Figure 1) featured here contains no unique or rare gemstones, nor is it especially valuable. The identification project described is, however, interesting for the many practical results and observations that bring together evidence from methods of general practice used by gemmologists to identify gemstones mounted in jewellery using only standard instruments.
The necklace provoked curiosity and led to background research about the gemstones: their colours and sources; their history; and their origin in this composite piece of jewellery. Some aspects were proven by testing, and some by inference and conjecture.
The necklace was probably made in the mid to early 20th Century using twenty different coloured gemstones of mixed mineral groups, species and varieties, all of modest value. Each gemstone is collet-set and linked to form a chain that originally had twisted gold wire sections in between each gem. The necklace, in its original form, was not marked with a gold standard. The original chain sections were replaced decades later in the 1990s and are now 9ct gold modern round trace links; the collets are original, made of unmarked gold which is ~15ct fineness by acid testing. As a result of changes to the links, the necklace is a ‘married’ item.
From the original necklace, the gemstones were kept in their same order, being mindful that the colours of the stones may have been assembled in order as an acrostic item or that the arrangement may have some other meaning. Depending on when and where the necklace was made, jewellery items can be set with this intention; for example, some traditional amuletic Indian jewellery in the form of pendant plaques, arm bands and bracelets (bazubands) were sometimes set with nine gems representing the planets (known as navaratna), and often worn as multiple bands (Krishnan, 2001). However, the planets are represented by a coloured suite of gems that include several that are not present in the suite of gems of this necklace, although gems are also known by some traditional and old names. Sentimental European jewellery may feature names and short messages represented by particular gems. It is more likely that this necklace is simply a collection of coloured stones; any message remains an enigma.
Figure 2. The complete necklace showing all gems in order from the clasp.
The Gemstones
There are twenty gemstones in total (Figure 2). All are irregular in the cutting style, most are approximately oval in outline and all are faceted, except one that is a biconvex deeply-cut asteriated cabochon. The dimensions of the gemstones clearly demonstrate that the intention of the lapidaries was to retain maximum weight with many stones having deep and bulging pavilions, and all are orientated to show the optimum colour through the table facet. The stones’ girdles are uneven, and many are thickly cut which assists in making them easier and safer to apply a collet setting. The outline of each gemstone is generally asymmetric with length to width ratios having no precise format, and the facets are a combination of irregular brilliant style of faceting mixed with extra facets, with none of the culets faceted.
The necklace has open settings, with both crowns and pavilions protruding from the settings thus exposing the gemstones to general wear and abrasion. The accompanying images of the individual gemstones show the considerable abrasion of facet edges over time. However, the necklace would have originally been assembled using simple handiwork and offered as an inexpensive item of jewellery with the buyer hardly aware of the mélange of gemstone varieties; in its naïve form it has colourful appeal. It is also a practical item of jewellery as in its original condition it required no clasp. Another benefit is its use as a portable reference collection of gemstones for gemmologists against which handheld spectroscope results and a range of natural inclusions can be compared.
The gemstones listed in Figure 2 are numbered from the clasp from 1 to 20 for reference, identified by mineral name and variety, with their dimensions, and their weights calculated from measurements with adjustment factors applied for the cut, to a total of approximately 25cts.
The equipment used in identification work included a standard gemmological microscope, dichroscope, bench refractometer, handheld diffraction spectroscope (with no facility for wavelength readings), short wave (SW) and long wave (LW) UV lighting and a petrological microscope. The petrological microscope is useful as it permits magnification up to x400 with polarizing filters, and can be used to examine and photograph specific inclusions. No advanced instrumentation was required for the gem identification work (but would be necessary to confirm the identities of mineral inclusions) and, as mounted gemstones, specific gravity testing could not be used.
All twenty gemstones are natural in origin with the colours of some zircons induced by heat-treatments. Of the twenty, there are nine zircons of various colours; four garnets of different varieties and colours; four sapphires comprised of one asteriated white (Geuda-type), one violet-blue and two dark green; two orange-brown tourmalines; and one purple coloured spinel.
Figure 3. Nine zircons in order as they appear mounted in the necklace. Gemstone weights, by calculation, are approximate.
Zircons
The colours of the nine zircons (Figure 3) include yellow to brown hues, light yellow to golden-yellow, orange-brown, reddish-brown, intense bright green, dark green, light blue and light greenish-blue.
Absorption spectra results show that all colours of the zircons, with the exception of the one blue specimen (gem 13), display the characteristic zircon band (653nm) in the mid-red spectral region. Most zircons also show diffuse multi band spectra with up to a maximum of eight bands which was the dark green zircon (gem 10). Gem 18, a yellow zircon, displayed the sharpest bands with a paired doublet in the red with an additional band in the far red and other sharp bands regularly spaced through the orange, yellow green and blue regions of the spectrum. The overall low number of absorption bands and their general diffuse quality is indicative of both the effects of heat treatment and metamict zircons. Characteristically, it is Sri Lankan zircons that display fewer bands than zircons from some other source countries. Blue zircons are very likely to have been heat treated as natural blue colour in zircons is rare, however, the responses of the two blue zircons were not the same.
Pleochroism responses of the zircons was varied from no detectable colour changes, some weak responses and some distinct. Colours that showed no detectable changes of body colours included yellow-gold, brownish-yellow (gems 1, 4, 7, 14 and 20) and dark green (gem 10). One yellow-brown zircon (gem 18) and both blue zircons (gems 3 and 13) displayed strong pleochroism with blue (ω-ray) and colourless (ε-ray) changes.
Fluorescence results demonstrated that most of the zircons are fluorescent with a strong yellow chalky surficial response under SWUV (gems 1, 4, 7, 10, 13, 14, 18 and 20) and were inert under LWUV. One blue zircon (gem 3) showed no fluorescence responses.
Zircons have high birefringence (ẟ0.059) displayed by strong doubling of features (DR) in faceted stones. This was viewed in most gemstones although not always observed through the table direction, but by tilting the gem, indicating the gem was likely cut for the table up view of its most intense colour or the greatest yield. Inclusions and test results of each zircon are described in Table 1.
Garnets
Four garnets form part of the necklace, one hessonite (gem 9) and three pyrope–almandine garnets (gems 11, 15 and 17). Three gems are rhodolite, a variety of the intermediate pyrope–almandine garnet ‘pyralspite’ isomorphous range with refractive indices of n1.746, 1.762 and 1.748 respectively. These show subtle colour differences from reddish-purple to pinkish-purple. All display a typical almandine ferrous iron absorption spectrum. Optical properties and inclusions for all these specimens are characteristic of garnet group gemstones (Table 2).
Table 1. Observations and test results for zircons.
Table 2. Observations, test results and refractive indices of garnets.
Figure 4. Zircon (gem 4) inclusions consist of a ‘ladder’ of small fractures. Note the extreme wear at the culet.
Figure 5. Images of four garnets. Gemstone weights, by calculation, are approximate.
Sapphires
Four sapphires of different colours and quality types form part of the necklace: three are faceted, one is blue-violet and the most valuable gemstone of the necklace (gem 2); two are dark green (gems 5 and 19); and one is a bi-convex cabochon, silver-white in colour and displaying six-rayed asterism (gem 12; Figure 9).
Figure 6. High resolution image of hessonite garnet (gem 9) with many mineral inclusions in view (actual gem size 6.7 x 5.5 x 3.4mm).
Figure 7. High relief mineral inclusions in hessonite garnet (gem 9).
Figure 8. General view of well-formed mineral inclusions in hessonite garnet (gem 9).
Figure 9. Images of four sapphires. Gemstone weights, by calculation, are approximate.
Tourmalines
The name tourmaline is reportedly derived from the Sinhalese word turmali, a generic name used historically in Sri Lanka for coloured stones, mostly zircons. Dutch lapidaries noted that some of these mixed gemstone parcels brought to Amsterdam from Sri Lanka in 1703 actually contained an undescribed mineral, which, by 1794, was eventually named tourmaline (www.mindat.org/min-4003.html; Herbert Smith, 1958). Historically, yellow tourmalines and zircons in Sri Lanka have both been referred to as ‘turmali’, and hessonite garnets and zircons of similar orange-brown colour were considered as having the same usefulness in jewellery by their colour rather than defining a mineral species.
Tourmaline is a super group mineralogical term and has many substantiated mineral varieties recognized by the International Mineralogical Association (IMA). Tourmaline is a complex borosilicate mineral and gemmological methods for the identification of tourmaline cannot provide the important detailed accurate chemistry to distinguish varieties as their colours are not species specific. New advanced methods of determining gem tourmaline species can apply laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA) to achieve some helpful results (Sun et al., 2019).
Two brownish-orange tourmaline gems (gems 6 and 16) form part of the necklace (Figure 12). Brownish-orange tourmalines could be from one of several varieties of tourmaline, including dravite, uvite and elbaite, and its variety is therefore not classified in this report.
Both tourmaline gemstones were identified from the results of refractive indices, obvious DR, and their distinctly strong dichroism. Unusually, both gems show bright surficial chalky yellow-golden colour fluorescence under SWUV. Fluorescence is uncommon in tourmaline group gemstones and records seldom quote luminescence reactions of gems from specific locales. Webster (1978) quotes a fairly strong yellow glow under SWUV of some tourmalines, including brown-coloured gems from Tanzania. A group of dravite tourmalines studied from Mozambique all had medium to strong green fluorescence under SWUV (254nm), a response not previously documented (Qin-Jing et al., 2019). The cause of the fluorescence responses has not been researched.
Table 3. Observations and test results of sapphires.
Table 4. Observations and test results of tourmalines.
Figure 10.Table view of blue-violet sapphire (gem 2) displaying shafts of white inclusions geometrically disposed. A booklet of brown mica is exposed on the table.
Figure 11. Profile of the sapphire displaying a deep pavilion.
Figure 12. Images of the two tourmalines mounted in the necklace. Gemstones weights, by calculation, are approximate.
Spinel
One spinel forms part of the necklace (Figure 13). A bright purple, transparent quality gemstone, referenced as gem 8, is identified as a spinel. It has a single measured RI of 1.715 and displayed no spectral absorption bands, no fluorescence and as an isotropic gemstone, no pleochroism. The inclusions consist of two internal fractures and a small group of minute zircon crystals that are elongated, prismatic ‘torpedo’-shaped and each with dark-coloured decrepitation haloes (Figure 14).
Figure 13. The purple spinel that is mounted in the necklace. Gemstone weight, by calculation, is approximate.
Figure 14. Group of zircon crystals in the pavilion of the spinel (gem 8), each with a discoidal stress fracture.
Discussion
The origins of the gemstones and the original source of the necklace are unknown but evidence of the common origin of these mixed species is that Sri Lanka (formerly Ceylon) is a likely country.
Sri Lanka has a history of over two thousand years as a source of many assorted gemstones and is long established for the use of heat treatment of gemstones. Hessonite and zircons of a similar colour are found together in Sri Lankan gem gravels (Herbert Smith, 1958). All the colours of sapphires including blue, green and colourless; spinel; rhodolite garnets; and tourmalines are also found in the prodigious gravels from various areas of Sri Lanka. For example, “Beautiful transparent stones, both brown and yellow accompany the green tourmaline in stone gravels in Ceylon as recorded at the turn of the 20th century” (Bauer, 1904), and “About 35-40% of colourless corundum, known as ‘Geuda’, mined in Sri Lanka belongs to this category.” (Mathavan, 2000).
Ceylon was “probably the original source of tourmaline as a gemstone, supplies stones of a yellow and brown colour from the alluvial deposits in the south-east of the island.” (Webster, 1978, p.129). Examples of zircon crystal inclusions with dark rimmed stress fractures are featured in Sri Lanka purple spinel in referenced literature (Webster, 1978).
The necklace provides an interesting project for the identification of fairly common gemstones by non-destructive standard gemmological testing and microscope observations, and as much as it would have been exciting to have discovered a rare gem species, the test results are conclusive for a well-known selection of gemstones. The author considered that having owned the necklace, purchased as a secondhand vintage item with a selection of well-worn gemstones, for some thirty-five years, the time had arrived for investigating its stone content. Unfortunately, no images exist of the original unstamped gold linkages.
Advanced instrumentation would succeed in providing specific information for each of the gemstones utilizing instruments that can provide results of the detailed trace element chemistry of the host gemstone and identification of the mineral inclusions. However, good information can be deduced using standard gemmological practice to obtain information using inexpensive portable equipment, non-destructive methods and with only the cost of time.
Summary
The naïve and rudimentary style of fashioning the gemstones, the mélange of stone content of the necklace with simple collet settings, is considered by the author to have been made in the early to mid-20th Century utilizing locally sourced Sri Lankan gemstones.
References
Bauer, M., 1904. Precious Stones. Charles Griffin and Co. Ltd: London.
Herbert Smith, G., 1958. Gemstones, 13th ed. Methuen and Co. Ltd., 36 Essex Street, Strand: London.
Krishnan, U., 2001. Jewels of the Nizams. Published by Department of Culture, Government of India: New Delhi in association with India Book House Pvt. Ltd.: Mumbai.
Mathavan, V., Kalubandara, S., and Fernando G., 2000. Occurrences of two new types of gem deposits in the Okkampitiya gem field, Sri Lanka. Journal of Gemmology 27(2), pp.65-72.
Qin-Jing, L., Wei-Zhi, H., Qian, Z. and Jing-Cheng, P., 2019. Gemological and spectral characterization of brownish yellow tourmaline from Mozambique. Spectroscopy and Spectral Analysis, 39(12),
pp.3844-3848.
Sun, Z., Palke, A., Breeding, C., and Dultrow, B., 2019. A new method for determining gem tourmaline species by LA-ICP-MS. Gems & Gemology, 55(1), pp.2017.
Webster, R., 1978. Gems, their sources, descriptions and identification. Butterworth & Co. Ltd: London.