Species and varieties
Quartz is silicon dioxide (SiO₂). The gem-grade members of the family are amethyst (purple), citrine (yellow to orange), smoky quartz (brown), rose quartz (pink), and rock crystal (colorless). Mohs hardness is 7; specific gravity is 2.65. Quartz is uniaxial and positive.
Sources
- Brazil — Minas Gerais and Bahia produce the bulk of the world's commercial amethyst, citrine, and smoky quartz.
- Uruguay — Artigas region produces deeply saturated amethyst geodes.
- Zambia — Kariba mine produces high-saturation amethyst; the source has grown in importance since the 1990s.
- Madagascar — produces rose quartz and smoky quartz at commercial scale.
- Switzerland and the Alps — historic source of clear smoky quartz crystals; small production today.
Treatments
- Citrine of commercial grade is typically produced by heat treatment of amethyst or smoky quartz precursors. The treatment is standard and is rarely declared as an enhancement on retail listings.
- Amethyst is occasionally heat-treated to lighten saturation; the practice is uncommon in the trade.
- Synthetic quartz (hydrothermal) has been commercially available since the mid-20th century and is identifiable by laboratory analysis of growth structures.
Lapidary considerations
Quartz is the entry-level lapidary species: hardness is moderate, the material polishes cleanly with cerium oxide, and rough is widely available in working sizes at low cost. Manufacturer cutting guidance for quartz-class material typically calls for a 600-mesh cutting lap, an 8,000-mesh pre-polish, and a cerium-oxide final polish on a felt buff or master lap.
The species is dichroic (notably amethyst, which shows blue-purple to red-purple pleochroism); orientation during preform layout affects the apparent color of the finished stone. Color zoning is common in Brazilian amethyst and is a layout consideration.