Tanzanite is the December birthstone and given on the 24th Wedding Anniversaries. It may be a relative newcomer to the world of coloured stones, but it was one of the most exciting gem discoveries of the 20th century. Blue stones emerging from Tanzania were identified as the mineral zoisite in 1962. The tanzanite birthstone is often described as “velvety,” mostly because of its deep and saturated colour, which ranges from a pure rich blue to violet, with the blue considered most valuable.