Topaz, one of the traditional birthstones for November and the zodiac signs Scorpio and/or Sagittarius, is a gemstone that you will normally encounter in its irradiated and heated form – the bright blue stones – or coated to create the iridescent effect of ‘mystic topaz’. Natural, untreated topazes come in an array of hues from colourless (at one time used as a diamond simulant as when cut well can show off some brilliance owing to its hardness of 8 on the Mohs’ scale), greens, yellows, oranges, pinks to incredibly rare purplish-reds. I saw a most spectacular one at the Museum of Sacred Art in Lisbon earlier this month, introduced by gemmologist Rui Galopim de Carvalho. The most well-known and signature hue of topaz is a deep golden-orangeish brown sometimes with pink known as Imperial Topaz and was once a favourite of the Russian Imperial court. It’s a super gemstone that can be cut in large, flawless sizes so is used as centrepieces for cocktail rings (also a benefit of its hardness and toughness – lapidaries just have to be careful to identify its axes as topaz exhibits perfect basal cleavage which means it is prone to fracturing along one axis of the crystal).
The name topaz is understood to derive from Topazos, an historical Greek-derived name for the island of Zabargad near Egypt in the Red Sea which was a source of peridot and other gemstones in the ancient world. Topaz was also used to refer to other yellowish-brown gemstones such as citrine, yellow sapphires and smokey quartz before it was identified as a distinct mineral species in 1737 by J.F. Henekel (Galopim de Carvalho, Gempedia, p. 656). The antiquity of the word is also attested by Biblical and other Judaeo-Christian descriptions of the High Priest Aaron’s breastplate, often referenced as an origin for the birthstone and zodiac system most in use today.
Don’t believe what you are told when it comes to gemstones
This is where my story about topaz ends. I am going to tell you what happened when I analysed a gem-set necklace from Sri Lanka that I was told contained all the colours of topaz including red ones which I did not know until later was an incredibly rare, and therefore highly valuable, colour. The seven-stone necklace, set in silver, with an integrated silver chain, was given to me by a relative from Sri Lanka who told me she believed they were topazes as she also had other similar stones in her jewellery. I loved this necklace so when I had the opportunity to learn and practice some of my gemmological skills during my Foundation in Gemmology with Gem-A, I took to analysing it myself to see what I could find out.
These were my initial observations.
There are seven stones, faceted ovals and transparent. The lustre of each is vitreous to bright vitreous (red and orange-pink stone). They are set in a an open-back semi-bezel four-claw setting and each of the settings are soldered together creating a solid pendant from which the thin chain is attached to either side. The setting and chain are silver.
The stones’ colours are from left to right: pale greenish-yellow, pink, pale yellowish-green, deep red, pale brownish-yellow, pink. The central stone is an orange-pink peachy colour. The faceting is quite poor and uneven though the facet edges and faces are relatively clean and sharp. There are a couple of points of minor abrasion where some of the facets meet.
I observed some very thin vertical inclusions in the central orange-pink stone as well as an inclusion which exhibited a flash of iridescence at a particular angle. I tried to clean off the dust but it was difficult to view through some of the paler green stones without being distracted by the many specs of dirt.
More or less the stones appeared quite clear of inclusions except that central one and the pale brownish-yellow one. On the central stone, the direction of what I thought was incipient cleavage is to the left of the table as you are looking down and it is in a vertical direction i.e. perpendicular to the plane of the table. On the pale yellowish-brown stone there is an inclusion parallel to one of the crown facets.
Tests. Each stone exhibited what I interpreted as a doubly refractive (DR) response with the Polariscope with light and dark rotating the stones 360 degrees. The dichroscope did not show much of a response for all stones except the central one which appeared brown and green.
At this stage I had not used the spectroscope as I was still grappling with it and fighting with my troublesome maglite. However my suspicions arose.
Beware of red stones on the polariscope
I took the necklace with me to the Gem-A Foundation Lab Class in May 2022. This is what I learned about the alleged topazes once tested with the handheld spectroscope.
The red and pink stones all showed the Fe (iron) Pyrope-Almandine spectrum. The Chelsea Colour Filter (CCF) colours were dark pink and dark red. The strength and width of the bands varied a little between the pink stones and the dark red one but all had easily observable thin bands in the blue-green, green and yellow to orange were moderately absorbed. I didn’t see anything in the red. Definitely garnets.
So what was going on with the polariscope when first tested? A very useful tip from the Lab Class tutors was to not entirely trust polariscope responses of red and pink stones. Anything that might be a pyrope-almandine series garnet has the potential to show ‘strain’, an aberration in the crystal which can cause a seeming ‘light to dark’ response on rotation through 360 degrees. The stones are small so counting the exact changes from light to dark is essential but also not in themselves always diagnostic with garnets which ought to appear dark throughout rotation because garnets are singly refractive (SR) being form the cubic crystal system.
The green and greenish yellow stones showed the U (uranium) Zircon spectrum with a clear strong line at 653nm. The CCF colours were paler versions of green and green to yellow.
The central brownish-pink stone which I was hopeful was a topaz, is also a Zircon and showed the U Zircon spectrum with a strong line at 653nm.
On closer examination, although these are not well-faceted or symmetrical stones, in good lighting I could observe some facet doubling suggesting the high birefringence of these stones although I struggled to see anything with the brownish-pink zircon. Interestingly I couldn’t observe any significant generalised facet abrasion consistent with some zircons but the nature of those little inclusions in the greenish-yellow stones may reflect zircon’s brittleness. My tutor then pointed out that the line at 653 nm is diagnostic and that Sri Lankan zircon tend to display an absorption line at 653.5 nm, 590 nm in yellow / orange and fainter lines in green and violet.
This piece of jewellery that’s been lying around for years has been such a mystery to me and now I feel that I have somewhat solved it and learned so many valuable lessons on the way. That they are garnets and zircons from Sri Lanka is just as interesting to me, if not more so. As zircon is one of the birthstones for December and garnet January, it still feels like an appropriate piece of jewellery to discuss in the growing depths of winter.