Clear Quartz

Clear quartz is the purest expression of silicon dioxide crystallization. It forms in an enormous range of geological environments - from pegmatites and hydrothermal veins to sedimentary deposits and metamorphic rocks. The key requirement is simply silica-rich fluid and enough time.

In hydrothermal veins, quartz crystallizes from hot, pressurized water as it cools below about 573 degrees Celsius (the alpha-beta quartz transition). Growth rates are exceptionally slow - a typical crystal point might grow at a rate of roughly one millimeter per thousand years. The largest clear quartz crystals, found in Brazil and occasionally in alpine fissures of the Swiss Alps, represent millions of years of uninterrupted growth.

Special varieties include phantom quartz (where growth paused, allowing a thin layer of mineral dust to deposit before growth resumed, creating ghost-like internal shapes), rutilated quartz (containing golden needles of the mineral rutile), and Herkimer diamonds (doubly-terminated clear quartz from Herkimer County, New York, formed in cavities within dolostone).

Clear quartz is identified by its glassy transparency, hexagonal prismatic crystal habit (six-sided prisms terminating in six-sided pyramids), hardness of 7, and lack of cleavage. It fractures conchoidally, producing the curved, shell-like breaks also seen in glass.

Distinguish from glass (which lacks crystal faces and may contain bubbles), clear calcite (which shows double refraction. Place it on text and you'll see doubled letters), and clear topaz (higher specific gravity, perfect cleavage). Clear quartz does not show double refraction and has a lower density than topaz. Natural crystal faces with horizontal striations across the prism faces are a reliable indicator of genuine quartz.