Of all the minerals, quartz is closest to a pure chemical compound and has constant physical properties. Crystals are often terminated by a hexagonal pyramid.  Quartz is the most common rock-forming mineral on the planet.  It is extremely resistant to weathering. It breaks down into sand grains which often form sedimentary rocks.

Cryptocrystalline varieties of quartz include agates and jaspers.  Colored varieties of quartz include AMETHYST, CITRINE, AMETRINE, ROSE QUARTZ, SMOKY QUARTZ and milky quartz.