Tanzanite

Tanzanite is the blue-purple variety of the mineral zoisite, found in a single deposit on Earth - a narrow strip of land approximately 4 kilometers long at the foot of Mount Kilimanjaro in Tanzania. This is the only known locality, making tanzanite one of the most geologically restricted gemstones in existence.

The deposit formed about 585 million years ago during the Pan-African orogeny - a massive mountain-building event that created intense heat and pressure in the regional metamorphic rocks. Zoisite crystallized in these conditions, incorporating trace amounts of vanadium (and sometimes chromium) that produce the blue-violet color.

Nearly all tanzanite on the market has been heat-treated. Raw tanzanite is typically brownish-burgundy; gentle heating to 500-700 degrees Celsius permanently shifts the color to the prized violet-blue by altering vanadium's absorption characteristics. This treatment is universal, permanent, and fully accepted by the gem trade. Rare unheated naturally blue stones command significant premiums.

Tiffany & Co. named the gem 'tanzanite' in 1968 and introduced it to the American market with a major advertising campaign. It was added as a December birthstone in 2002.

Tanzanite shows strong pleochroism - it appears blue, purple, and burgundy from different viewing angles. This trichroism is one of its most distinctive features. At hardness 6.5, it's softer than sapphire (which it can resemble) and has one direction of perfect cleavage that makes it vulnerable to hard impacts.

Distinguish from blue sapphire (much harder at 9, no trichroism), iolite (lower specific gravity, different blue), and blue spinel (no pleochroism, different crystal system). The violet component in tanzanite's blue is distinctive - it's warmer and more purple than sapphire's pure blue.