Birthstone of the Month: December

It is time once again for a look at birthstone of the month, which for December is Tanzanite. This beautiful transparent blue to violet gemstone is rarer than diamonds, due to its more recent discovery and limited supply.

5.43ct tanzanite, diamond and 18ct white gold ring, available at Buck & Co, Bodmin, Cornwall

​A variety of the mineral zoisite, it was discovered in Tanzania in 1967. Tiffany & Co launched it onto the commercial gemstone market with the name tanzanite to reflect its geographical origins (rather more poetic than 'blue zoisite' or the chemical name of calcium aluminum silicate) with the tag line: "Tanzanite can now only be found in significant quantities in only two places in the world. In Tanzania. And Tiffany's".

A tanzanite faceted gem (or rough crystal) may be seen to exhibit the apparently magical property of appearing to be different colours when viewed from different directions! This is seen clearly in this stunning example from our Buck & Co stock, where the effect is so pronounced it can be seen with the eye, no jewellers loupe required! This 5.43ct tanzanite when viewed from one side appears blue, but when viewed from the other side appears purple, and it is this combination of colours that creates the bluish-purple saturation that makes tanzanite so unique and so sought after.

Although this appears as an otherworldly gemstone quality, it is due to the science of how light travels through the stone, and is known as pleochrosim. When a light ray passes into a doubly refractive gemstone it is split into two directions which vibrate at 90 degrees to each other and at different speeds. This means that the light rays are absorbed differently and show as different colours in different directions. Tanzanite can exhibit up to three different colours but they will never be seen all at one time, and stones are often heat treated to remove the third greenish yellow to brown colour. A gemmological tool called a dichroscope can be used to view two of the pleochroic colours simultaneously.

This particular tanzanite is from our secondhand collection, and is set in 18ct white gold throughout and is surrounded by diamonds, the whiteness of which set off the deep saturation of the tanzanite perfectly. It is currently a size O (but can be resized by our in-house Goldsmith) and has full UK hallmarks. Viewings are highly recommended to experience the natural beauty of this magical pleochroic gemstone for yourself!