What causes the purple color of amethyst?

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The color in amethyst comes from color centers in the quartz. These are created when trace amounts of iron are irradiated ( from the natural radiation in the rocks). The purple color in ghost town glass comes from small amounts of manganese in the glass when it has been exposed to ultraviolet light. The manganese was used as a clarifying ingredient in glass from 1860 to 1915. Prior to that, lead was used, and subsequently, selenium is used. Two books that have sections on the creation of color in quartz are the third volume on silica in Dana's System of Mineralogy ( the seventh edition in three volumes). The MSA Reviews in Mineralogy had a chapter on color formation in volume 29 Silica ( an excellent explanation of the various color causes in quartz). Quartz will commonly contain trace amounts of iron ( in the range of 10's to 100's parts per million of iron). Some of this iron sits in sites normally occupied by silicon and some is interstitial ( in sites where there is normally not ... more
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