Generally, Tanzanite (or Zoisite) is found in a reddish brown that is heat-treated to turn the stone sapphire blue, amethyst or violet. Heat treatment deepens the blue color saturation but doesn ...
And the gemstone is as British as it gets. Jet is one of only two gems mined in England — the other being Derbyshire blue john, which resembles amethyst streaked with dark veins and bands.