GIA Gemologist Article Series on Tanzanite
Tanzanites are rare and beautiful and come in a variety of colors. they also change their appearance markedly in different lights. This article explores these remarkable tenets.
Blue Tanzanite - Blue Tanzanite is the most widely known. It comes in a wondrous range of blues and violets but it is also very sensitive to the light source it is viewed under, so the same stone can appear very different in varying circumstances. Anyone who has owned a Tanzanite will know how much of a “mood changer” this gem type is. During the day, in the glare of the white fluorescent strip lights of the office environment, your pendant glows a deep blue and then at night in the restaurant with the low lighting and predominance of incandescent lighting, your friends comment on how the sparks of red in your Tanzanite are so eye catching.
So why is this so? Tanzanite is a trichroic gemstone. This means that Tanzanite’s chemical structure causes the double refraction of white light as it passes through it. A good illustration is in looking at a Tanzanite crystal in its rough form, turning it on 3 different axes you can perceive 3 different colors. One axis will appear blue, another reddish brown and the other a deep violet.
Same Tanzanite crystal from 3 different angles. Illustrating Tanzanite's trichroism Photo Lapigems 2011
It is this trichroism which is responsible for the way your tanzanite behaves under different lighting conditions. Selective absorption of different color components in the light source cause the stone to appear completely different depending on the light it is being viewed under. In white light, like fluorescent strip bulbs commonly found in offices, the stone absorbs more of the long wavelengths (reds, oranges) and emits shorter wavelengths (blues). In incandescent (warmer light sources) it absorbs more of the shorter wavelengths (blues) and emits longer ones which is why the pinks and reds are visible like in the photographs below of the same piece taken in different light sources.
In White Light & Incandescent light. Illustrating Tanzanite's trichroism Photo Lapigems 2011
To read more about this phenomenon read our article “Gemstones and Lighting”.
The gem species from which the variety name “Tanzanite” derives is called Zoisite which is a member of the Epidote group. This group also produces various colors other than the well known blue. The different colors are caused by different trace elements within the chemical structure. In blue Tanzanite, the color is caused by the presence of Vanadium. However, other trace elements within a crystal can cause other colors. Manganese for example results in a pink, whilst chromium produces a green.
There are a variety of more “undesireable” colors also produced, with too much brown and or grey secondary hues resulting in a "dirty" hue. The majority of fancy colors mined are of this quality. Sharp pinks, greens yellows and oranges are rare and can be very beautiful and coveted. Most stones are very small and larger pieces are very rare, especially in purer colors. The other boon is that these colors are not heated but occur naturally.
Various colors of Fancy Tanzanite Rough. Stones seen and photographed in Lapigems offices
These colors are mostly purchased by collectors or investors as the mainstream market is mostly unaware of their existence.
This series of GIA Gemologist written articles on Tanzanite is provided to create a greater understanding of Tanzanite value and quality in the face of increased misinformation on the internet today. Others in the series are listed below. Click on each to read them:
Tanzanite Color How it Affects Value
Tanzanite Cut Quality - Often Overlooked but Very Important
Tanzanite Color Grading Systems - Which to Trust?
Tanzanite Clarity - How it Impacts on the Grade
Tanzanite Trichroism - The Mood Changer
How to Care for Your Tanzanite
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