Quartz and Agate - SiO₂

Habit: Colorless, white, rose pink to rose red, yellow to yellowish brown, green, blue, bluish violet or brown to black. Typically long prismatic crystals with steep pyramidal terminations, but may be short prismatic to nearly bipyramidal; fibrous for agate and chalcedony. Vitreous luster, but waxy to dull when massive; transparent to nearly opaque. White streak.

Environment: The most common mineral found on Earth’s surface and a significant component of many igneous, metamorphic and sedimentary rocks.

Etymology: Of obscure origin in the Middle Ages, the name quartz was first applied to gangue, or waste rock, in Saxony (now part of Germany). Agate, which is a form of microcrystalline quartz — chiefly chalcedony — was named by Theophrastus, a Greek philosopher and naturalist, who discovered the stone along the shoreline of the river Achates (now called Dirillo) in Sicily.

Agate is a common type of chalcedony, which is a compact, microcrystalline variety of quartz. Much of the sliced agate on the market today, particularly the brightly colored examples, has been dyed or stained to enhance its natural color. Quartz is one of the most ubiquitous minerals on Earth’s surface, forming in almost every geological environment and in a huge variety of shapes, sizes and colors. The most common varieties of quartz are amethyst (purple), citrine (yellow), rose quartz (pink) and smoky quartz (brown to smoky gray to black).