2023

Purple love

By Dr. M.S. Krzemnicki, first published in Facette 28 (May 2023)

 

It is always a pleasure for our team to analyse gemstones which are not so common, or sometimes even very rare, but which may compete in beauty and quality (but often not in hardness) with the most prestigious classic gemstones. In the past few months, we had the opportunity to analyse a number of rare and outstanding gemstones of purple colour, some of which are presented in the following.

Figure 1: Hackmanite of nearly 9 ct showing a vivid purple colour and a fine clarity. Photo: A. Chalain, SSEF.

The first example is a transparent faceted hackmanite from Afghanistan of nearly 9 ct. Hackmanite, a variety of sodalite, is known for its striking colour shift from whitish grey to intense purple when exposed to ultraviolet light. Normally, this colour subsequently fades again over time, but can be re-established indefinitely. This property is known in scientific literature as tenebrescence, or reversible photochromism.

Figure 2: Oriented needle-like inclusions in the described hackmanite. Photo: M.S. Krzemnicki, SSEF.
Hackmanite used as gem material is commonly quite included and of low transparency and thus often cut into cabochons or beads. The described hackmanite submitted recently to SSEF is however a very fine example in size and clarity for this gemstone variety. It showed an interesting three- dimensional pattern of needle-like inclusions (Figure 2) and some small fissures filled with a minor amount of oil.
Figure 3: our diaspore samples from Afghanistan ranging in weight from 26 ct (left) to 7 ct (right) analysed recently at SSEF. Photo: A. Chalain, SSEF

Diaspore, with a chemical formula of AlO(OH), is known in the gem trade since decades from the Milas District, Mugla Province, in SW Turkey (e.g. Schmetzer & Bartelke, 1979). These stones show an interesting colour change from olive green in daylight to brownish in incandescent light. Purplish pink diaspore has been found first in limited quantities in Mong Hsu in Myanmar (Kyi & Win, 2004) and recently in larger sizes and quantities in the Nangarhar Province in eastern Afghanistan (Nicastro et al., 2020). SSEF received a series of four diaspore for testing (Figure 3), all of similar purplish pink colour and of outstanding clarity. The stones showed a distinct red fluorescence in longwave ultraviolet light due to traces of chromium, and a distinct yellowish chalky reaction under shortwave ultraviolet light, as characteristic for this new material from Afghanistan (Nicastro et al. 2020, Milisenda & Wild 2021). Chemical and absorption spectroscopy analyses confirmed the combination of traces of chromium and vanadium, as being mainly responsible for the purplish pink colour of these stones.

Figure 4: Exceptional taaffeite of 39 ct analysed recently at SSEF. Photo: L. Phan, SSEF.

One of the most outstanding purple gems encountered at SSEF in the past few months was definitely an exceptional taaffeite of 39 ct, reportedly originating from Sri Lanka (Figure 4). Known and sought after as a collector stone, taaffeite, ideally BeMg3Al8O16, was named after Mr Richard Taaffe, who by chance discovered the first specimen of this mineral in 1945 in a jewellery shop in Dublin (Ireland). Due to its visual appearance, the specimen was offered to him as a spinel and only after his lucky discovery described as a new mineral species. Taaffeite forms a mineral group together with musgravite, another exceedingly rare mineral and gemstone from which it can only be separated by sophisticated analysis. Detailed Raman spectroscopic analyses and trace element analyses (GemTOF) clearly revealed that the described gemstone is a taaffeite, owing its attractive purple colour mainly to iron, similar to purplish spinel (Andreozzi et al. 2019, Lhuaamporn 2022). Interestingly, chromium which is known as a contributing colour cause in pink to purple taaffeite (see also Facette 2021, No. 27, pages 14-15) is present only at sub-ppm level in the described gemstone, thus certainly not contributing to its purple colour. When exposed to longwave ultraviolet light, this taaffeite exhibits a weak green fluorescence, similar as observed occasionally in spinel (Krzemnicki 2023) and commonly attributed to traces of manganese (about 800 ppm Mn in this specimen).

Figure 5: A spinel with a 6-rayed star effect. Photo: L. Phan, SSEF.

The last purple specimen described in this short note is an exceptional spinel with an attractive 6-rayed star effect submitted recently to SSEF (Figure 5). The star effect, also known as asterism, is a reflection effect on tiny oriented inclusions dispersed in the gemstone. Its visibility and pattern (e.g. 6-rayed or 4-rayed) after having been cut into a cabochon is directly linked to the amount, shape and orientation of these tiny inclusions, often small needle-like solids of fluid-tubes.

The described purple star spinel is outstanding in size and weight (40 ct) and has been perfectly cut to show a well-centred 6-rayed star (Figure 5). When rotating the stone in all directions, similar 6-rayed stars are visible at the end of each of the six branches, indicating a complex three- dimensional pattern of inclusions to enable such multiple star effects within one single stone (Figure 6). In addition, our analyses revealed that the colour of this purple star spinel is related to a combination of iron and chromium.

Figure 6: Star effect seen at the end of one of the 6 branches (left), and dense three- dimensional pattern of oriented needle-like inclusions in this star spinel, resulting in multiple star effects on this cabochon. Photos: L. Phan & M.S. Krzemnicki, SSEF
Light reflection effects on crystal spheres or cabochons have fascinated mineralogists and gemmologists for more than a century (see e.g. Goldschmidt & Brauns 1911: Über Lichtkreise und Lichtknoten an Kristallkugeln, Neues Jahrbuch Min. Geol. Paläntologie). Similar complex multi-star effects as observed in this purple spinel were already described in gem-quality almandine garnet and quartz (Schmetzer et al. 2002, Schmetzer & Glas 2003, Schmetzer & Krzemnicki 2006). To see this effect so beautifully exhibited in a spinel of this size can be definitively considered rare. The aim to present in this short note this exclusive selection of rare and uncommon purple gemstones was not only to describe their specific gemmological properties, but also to show their beauty and quality, which make them attractive gemstones to fall in love with for gemmologists, collectors and gem aficionados.