Quartz is one of the most common minerals on the planet, providing everything from the sparkle in granite to the ticking of wristwatches—the stone’s vibrations are regular enough to keep time. Rock crystal quartz is colorless, but amethyst is purple, citrine is yellow, and aventurine is green. All are varieties of gem-quality, single-crystal quartz. Gem-quality cryptocrystalline (also called microcrystalline) quartz is also called chalcedony,...Continue Reading