Amethyst

The color in amethyst from most localities is unevenly distributed in the individual crystals. In amethyst geodes, it is often most intense in the growth zones under the rhombohedral faces (at the tips). Occasionally the color is deeper under either the r or z rhombohedral faces, giving the crystal a pinwheel appearance when viewed from the top. In prismatic crystals, the colo r may appear in phantom-like thin layers, while in scepters and skeleton quartz the color is often concentrated along the edges, and accompanied by smoky zones. Despite the intense color, the content of iron occupying Si positions in amethyst is rather low, in the 10-100 ppm range (Dennen and Puckett, 1972). When heated to more than about 300-400°C, amethyst loses its violet color and often turns yellow, orange or brown, and then resembles the quartz variety citrine, but depending on the locality and the temperature during the heat treatment it may also turn colorless or - rarely - green (Rose and Lietz, 1954; Neumann and Schmetzer, 1984). Irradiation with UV light will also destroy the color centers, and accordingly prolonged exposure to sunlight will slowly fade amethyst (Currier, 1985). The photo to the right shows the effects of heat (bottom left and right) and UV irradiation (top right) on the color of a specimen from Uruguay.
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