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| Considering the worldwide distribution of these common substances, it is remarkable that gemstones are so rare. Apart from the requirement that, in the first place, factors promoting crystallization must be present, in many gemstones of simple composition it is the extremely rare coloring agent (e.g., chromium in alexandrite, ruby, and emerald) that determines the distinct rarity; in gemstones of complex composition, on the other hand, rarity is occasioned by an important but minimally represented constituent such as fluorine in topaz and boron in tourmaline. |
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