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Crystals. Selection of uncut crystals. These solids have a polyhedral (many-faced) shape. The regular arrangement of crystal particles ensures that all crystals of the same substance possess the same angles between their faces. The external appearance is not necessarily the same however, as the different faces may grow at different rates. Some crystals, when polished and cut, are used as gemstones. (KEYSTONE/SCIENCE PHOTO LIBRARY/VICTOR DE SCHWANBERG)