From The Gemology Project
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Chemical composition SiO2 Silicon dioxide
Crystal system Trigonal
Habit Prismatic
Cleavage Poor
Fracture Conchoidal
Hardness 7
Optic nature Uniaxial +
Refractive index 1.544 - 1.553
Birefringence 0.009
Dispersion 0.013
Specific gravity 2.65
Lustre Vitreous
Pleochroism None

Quartz is a common variety of silicon dioxide that often crystallizes into well formed hexagonal prisms. It occurs in many colors, purple amethyst, yellow citrine, and a smoky variety, referred to as smoky quartz or Cairngorm. The colorless, transparent crystals represent the modern concept of "crystal". Crystal balls with a truly mystic significance are always made from quartz. Quartz crystals are among the earliest talismans. Examples of rock crystal objects date back to 75,000BC. Every culture on the planet has attributed magical power to quartz crystals. The Japanese considered it "the perfect jewel", symbolizing patience and perseverance, the infinity of space and purity. Native Americans used quartz crystals as a hunting talisman and "fed" them with deer's blood. Crystal balls were brought to Europe from the Near East by the Crusaders. They were credited with the ability to cure diseases. In Scotland and Ireland, farmers used crystal balls to keep livestock healthy.