It is found in various forms and shapes, the most common growth forms are:ġ. The strength of the effect varies to a considerable degree, and changes in the hue depending on the direction of the transmitted light may be observable with the naked eye, in particular in crystals with a zonar development of color, which may even show sky-blue tones.Īmethyst crystals do not get very large, crystals longer than 30cm are very rare. 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.Īmethyst is pleochroic (Haidinger, 1847 Pancharatnam, 1954 Raman, 1954): when the polarization of the light is changed from parallel to the c-axis to perpendicular to the c-axis, amethyst changes its color from blue-violet to purple. Irradiation with UV light will also destroy the color centers, and accordingly prolonged exposure to sunlight will slowly fade amethyst (Currier, 1985). When heated to more than about 300-400☌, 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). 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). In prismatic crystals, the color 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. 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 amethyst geodes, it is often most intense in the growth zones under the rhombohedral faces (at the tips). The color in amethyst from most localities is unevenly distributed in the individual crystals. Changes in color of amethyst by heat treatment and UV radiation.
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