GIF# 16 Quartz (rock crystal pictured)
Named after a Slavic word for “hard” it is the name used for a group of minerals of the same chemical composition (SiO2) and similar physical properties. Microcrystalline quartz (crystals recognisable with the naked eye) includes stones gemologists classify as varieties of the quartz species: amethyst, aventurine, rock crystal, blue quartz, citrine , smoky quartz, rose quartz and tigers eye.
Cryptocrystalline quartz ( microscopically small crystals) generally known as chalcedony, includes agate , petrified wood, chrysoprase, bloodstone, jasper and carnelian.
MOHS Hardness average= 7