Geology Home Page
Roger Weller, geology instructor
Chemical Group: oxide
Chemical Formula: SiO2
Color: colorless, white, pink, purple, brown, red, yellow, black, etc.
Specific Gravity: 2.65
Cleavage: none Luster: vitreous (cryptocrystalline quartz ranges from waxy to dull)
Crystal Forms: Trigonal
Crystals are commonly hexagonal prisms, capped with 3 large rhombohedral faces
and three small rhombohedral faces.
Quartz also forms twinned crystals; the most famous are Japanese twins where
two flattened quartz crystals are attached at right angles to each other.
Special properties: piezoelectric and pyroelectric
Mineral Associations: Quartz is found where aqueous solutions have deposited it
in fractures and volcanic vesicles.
Identifying Characteristics: hardness 7, resistant to chemical weathering
Uses: gemstones, making glass, abrasives
Occurrences: just about everywhere. Nice crystals come from Brazil and Switzerland;
in the USA from Arkansas, Colorado, New York, Idaho, New Mexico
Toxicity: when-swallowed- low when inhaled- high
Transparent quartz gemstones: amethyst (purple), citrine (yellow),
smoky quartz (brown to black), rock crystal (colorless).
ametrine (purple and yellow)
Translucent gemstones: agate, aventurine, chalcedony, chrysoprase,
jasper, tigereye, bloodstone, carnelian, sard, onyx
Other common forms: milky quartz, chert, flint, rose quartz