THE AUSTRALIAN GEMMOLOGIST | Zoisite – a Gemstone of Many Colours
Zoisite – a Gemstone of Many Colours
Figure 1. A 7.5mm trilliant cut unheated zoisite weighing 1.34ct and a 7.5mm trilliant cut heated tanzanite weighing 1.38ct, both from Tanzania.
We all know the common narrative of tanzanite being first found in Tanzania in 1967 and named by Tiffany and Co’s Henry B. Platt after the gemstone’s geographic origin. This widely used trade name only refers to the purple and blue variety of the mineral zoisite1. But what do we know of the beauty of other coloured zoisites? Most people would believe that zoisite is a brown or golden colour best suited for heating to produce the illustrious purple and blue tanzanite (Figure 1). Or perhaps it is an especially obvious trichroic specimen that shows blue, purple and green down different crystallographic axes? Most tanzanite on the market has been heated to remove these shades of brown, golden and green colours which are considered undesirable; however, there are other gorgeous colours of zoisite from Tanzania that are lost to heating in favour of blues and purples.
Figure 2. A 2.14ct unheated Tanzanian hot pink/reddish-orange fancy zoisite crystal found during a buying trip that changed the author’s view about unheated zoisite forever.
Fancy-Coloured Zoisite
As a lapidarist in search of beautiful colours and unusual gemstones to cut, it shocked me to see my first hot pink zoisite (Figure 2) in 2020 when I was on a gem sourcing trip. Since then, I have made it my mission to find other unusual or unknown colours of zoisite which are available commercially before they are heated to be sold as tanzanite. Over the past 2 years I have found a rainbow of colours of unheated zoisite, often labelled as fancy-coloured zoisite. These colours range from soft pink, peach and magenta, to yellow, lime yellow, cognac, brown, and in rare cases, even show colour zoning (Figure 3).
Figure 3. A colourful range of natural, unheated zoisite rough from Tanzania with some examples showing colour zoning.
Pleochroism and Crystal Orientation
While in the rough these colours can be quite spectacular to behold, knowing how to orientate them to showcase the most favourable axis of colour can often be quite difficult or unpredictable. Working with any strongly trichroic gemstone needs considerable experience and knowledge to bring out the best colour and shape without losing too much weight. Quite a few of the fancy-coloured zoisites that have pink showing down one axis, often have yellow or cognac down another axis; after cutting, the final gemstone will show a combination of these pleochroic colours when looking through the crown. Sometimes this blending of pleochroic colours in the finished stone can result in a far less favourable murky purplish-brown which would have been better suited to heating (Figure 4). However, if orientated correctly and sometimes with more weight loss, the colours across different axes can blend nicely, and a highly sought-after pink can be achieved (Figure 5). In both examples shown in Figures 4 and 5, these cuts were chosen to maximise the weight of the faceted gemstone, follow the shape best suited from the rough crystals, while removing or minimising visibility of any inclusions.
Figure 4. An unheated fancy zoisite from Tanzania. Left: showing purplish pink in one crystallographic direction. Centre: showing a brownish-peach colour in another direction of the crystal. Right: the finished 5mm gemstone weighing 0.67ct after cutting.
Figure 5. An unheated fancy zoisite from Tanzania. Left: showing a yellowish colour in one crystallographic direction. Centre: showing a hot pink colour when looking directly down the C-axis. Right: the finished 7.9 x 5.7mm gemstone weighing 1.38ct after cutting.
Rarity and Desirability
While tanzanite is already regarded as rare and desirable in high-end jewellery, these fancy-coloured zoisites from Tanzania are beginning to find their own place in the market of high-end jewellery, sometimes fetching a higher price than tanzanite, depending on their colour, size and beauty (Figure 6). Saturated pinks are at the higher end of the scale as they are rarer and more sought after; yellow or pale lime green zoisite is a far less desirable colour as there are many other similarly coloured gemstones in the market at a cheaper price point, such as chrysoberyl, peridot and quartz. Zoisite that displays cognac or golden-brown tones in the rough are the least desired in their unheated colours, and no faceted gemstone design can fully hide those tones from showing in the finished stone. Most often, cognac or golden brown zoisites are best suited for heating to fetch a higher price as they are then marketed as tanzanite.
As a gem cutter and a gemmologist, I have found working with all the different colours of unheated zoisite to be rewarding, as there are very few gems that have such distinct trichroism. Orientating three directions of colour can be both a challenge and unpredictable and having cut over 40 specimens of zoisite and tanzanite in the last two years, no two stones have turned out alike in their final colour. These fancy colours of zoisite, often lost to heating in favour of blues and purples, have their own unique place in the market. Not every stone needs to be heated or treated to bring out their beauty, and with the latest shift amongst consumers for natural, untreated stones I have seen more of these available from artisanal gem dealers.
All stones cut by, and all photos courtesy of, the author.
Figure 6. A varied range of faceted Tanzanian unheated zoisite and heated tanzanite.
Footnotes
1 Some reference material and commercial suppliers refer to all gem zoisite as tanzanite, primarily as a marketing tool.
However, CIBJO and GAA Notes (2021) define tanzanite as blue to violet, and transparent other colours as zoisite with a colour prefix.
