Birthstone
Tanzanite
Usually described as violet-blue.
There is no plate for tanzanite, and there could not be one. It was found in 1967 in the Merelani Hills below Kilimanjaro and named the following year — the age of the hand-coloured mineral plate was long over, and it is the only birthstone on either list that no Victorian mineralogist ever saw.
- Colour
- violet-blue
- Month
- December
- Status
- Added laterTanzanite was added to December in 2002 by the American Gem Trade Association. The two lists compared.
Name and history
The name dates from 1968 and is said to have been given at Tiffany & Co., after Tanzania, the country where the stone was discovered.
What it actually is
| Species | Zoisite |
|---|---|
| Composition | Ca₂Al₃(SiO₄)(Si₂O₇)O(OH) |
| Crystal system | orthorhombic |
| Hardness | 6–7 on the Mohs scale. At best as hard as the quartz in household dust, and usually softer, so it dulls with everyday wear. |
| Cleavage | perfect in one direction ({010}), imperfect on {100} |
| What makes the colour | vanadium (V³⁺) replacing aluminium; the crystal is strongly trichroic, showing blue, violet and a brownish-yellow to green along its three axes, and heating destroys the brownish component |
| Mined in | Tanzania |
Hardness, composition and crystal system are standard mineralogical values. Where published sources disagree, the range given covers them.
How it gets damaged
One perfect cleavage combined with only moderate hardness makes it easy to chip or split, and it is unusually sensitive to sudden temperature change; never use an ultrasonic or steam cleaner, and set it where it will not take knocks.
What is routinely done to it
Heat treatment is effectively universal — it removes the brownish-yellow pleochroic colour and leaves the blue-violet; because every stone is assumed treated, it carries no price penalty.
This describes the physical stone. Nothing on this site claims a gemstone has any therapeutic or healing effect — see the disclaimer.
Which list it is on
| Month | The 1912 list | Today's list |
|---|---|---|
| December | — | Yes |