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Diamonds, rubies and other gems are valued for their sparkle and beauty—but also for their rarity. It takes an exceptional convergence of geological conditions to create gemstones, which, as the author explains, has drawn increasing numbers of earth scientists to study gems and their origins. For instance, emeralds form where rocks from deep within continents encounter minerals from ocean ridges. Groat details the formation and chemistry of the major gemstones, along with new research that aims to quantify the most likely geological formations where different gems might be found.