Species and chemistry
Chrysoberyl is beryllium aluminum oxide (BeAl₂O₄). Mohs hardness is 8.5 — third among gem-grade species after diamond and corundum. Specific gravity is 3.70 to 3.72. Chrysoberyl is biaxial and positive.
Varieties
- Chrysoberyl — common variety; greenish yellow to yellow.
- Cat's-eye chrysoberyl (cymophane) — chatoyant variety with a sharp single eye produced by oriented inclusions; cut as cabochon.
- Alexandrite — chromium-bearing variety with a marked daylight-to-incandescent color shift, typically green to red.
Sources
- Brazil — Hematita, Minas Gerais: source of alexandrite with strong color change.
- Sri Lanka — historic and continuing source of chrysoberyl and cat's-eye material.
- Russia — Ural Mountains, Tokovaya River: historic source of fine alexandrite (the type locality, now substantially exhausted).
- Madagascar — alexandrite and chrysoberyl in commercial production since the 1990s.
- Tanzania — Lake Manyara region: alexandrite and chrysoberyl.
- India — Andhra Pradesh: alexandrite of variable color change strength.
Treatments
Chrysoberyl and alexandrite are generally untreated. The exceptional rarity of fine alexandrite has driven commercial development of synthetic and simulant material:
- Synthetic alexandrite (Czochralski, flux, hydrothermal) is commercially available and is identifiable in the laboratory.
- Color-change synthetic corundum — produced for jewelry use and sometimes misrepresented as alexandrite — has a different color-change profile distinguishable by spectroscope.
Lapidary considerations
Chrysoberyl is hard and tough; the species cuts cleanly at standard diamond grits. The orientation of alexandrite affects the strength of the color-change effect: the c-axis orientation typically produces the strongest shift. Cat's-eye material is cut as cabochon with the chatoyancy axis perpendicular to the dome length.
Final polish is conventionally performed at 50,000 mesh or with a tin-lead master plate. The species takes a clean optical finish.