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Chalcedony, mineral consisting of microcrystalline silica (silicon dioxide) with a glassy or waxy luster, sometimes translucent. Chalcedony occurs in a wide range of colors, some forms valued as gems and ornamental stones. Bloodstone (heliotrope), a dark green variety with red spots, was often used in the Middle Ages in sculptures showing martyrdom. Other chalcedonies are jasper, carnelian, agate, and onyx.

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