Tsavorite, the Tsavo garnet

It is named after a Kenyan national park, and it was found in Tanzania.

Tsavorite, sometimes tsavolite, is the green variety of grossular, a calcium-aluminium garnet. Trace vanadium, principally, with some chromium, produces the colour. At its best it rivals emerald, and it has become one of the most popular and expensive garnets there is.

The discovery took two countries and four years. In 1967 the British gem prospector and geologist Campbell R. Bridges found green grossular in the mountains of the Simanjiro District of Manyara Region in north-east Tanzania, at a place called Lemshuko about 15 km from Komolo, the nearest village. The stones were intensely coloured and highly transparent, the trade took notice, and then the Tanzanian government declined to issue export permits. Bridges reasoned that the deposit was part of a larger structure that might continue into Kenya, went prospecting there, and in 1971 found it again and was granted a permit to mine.

For years it stayed a specialist's stone. That changed in 1974, when Tiffany and Co. launched a marketing campaign that put it in front of the public, which is also how it got a name taken from Tsavo East National Park in Kenya rather than from the Tanzanian hills where it first appeared.

The story ends badly. Campbell Bridges was killed in 2009 when a mob attacked him and his son on their property in Tsavo East National Park, after a long dispute over the ground.

For anyone buying rather than digging, tsavorite behaves well. Like most garnets it stands up to daily wear, works in engagement rings without needing a protective setting, and takes a wide range of faceting designs. Unlike emerald it is not routinely treated, which is a large part of why gemmologists like it. The mines themselves are commercial operations on restricted ground, so this is a stone to buy, not to prospect for.