Species and varieties
Topaz is aluminum silicate fluoride hydroxide (Al₂SiO₄(F,OH)₂). Mohs hardness is 8; specific gravity is 3.49 to 3.57. Topaz is orthorhombic with a perfect basal cleavage that bears directly on lapidary work.
Natural colors range from colorless through pale yellow, brown (sherry), pink, and rare blue. Saturated blues on the contemporary market are produced by treatment of colorless or pale-blue precursors. The "imperial topaz" trade term describes a saturated golden-orange to sherry-red material associated principally with the Ouro Preto, Brazil, locality.
Sources
- Brazil — Ouro Preto in Minas Gerais: the source of imperial topaz, in continuous production since the 18th century.
- Brazil — broader Minas Gerais: colorless and pale-blue topaz in commercial volume.
- Nigeria — Jos Plateau: pale-blue and colorless precursor for treated material.
- Pakistan — Katlang in Mardan district: pink topaz of moderate saturation.
- Russia — Urals: historic source of pink and sherry topaz.
- Ukraine — Volyn region: clear and smoky topaz.
Treatments
- Irradiation followed by heat is the standard treatment for the saturated "Swiss blue", "London blue", and "sky blue" material on the market. Colorless topaz is irradiated, then annealed at moderate temperature to develop and stabilize the blue color. Treatment is detectable by sufficient post-radiation lifespan; residual radioactivity is monitored before retail release.
- Heat treatment is used to alter sherry to pink in some Pakistan and Russian material. Treatment is generally disclosed.
- Coatings — surface coatings producing iridescent ("mystic topaz") effects — are common in lower-end retail. The treatment is impermanent and is generally disclosed.
Lapidary considerations
The single perfect cleavage in topaz is the principal lapidary consideration. The cleavage plane is perpendicular to the c-axis. Cutters orient the preform so that no facet is parallel to the cleavage at high angles, and they avoid percussive contact during dopping and cutting.
Topaz is dichroic; orientation affects the finished stone's color. The species cuts and polishes cleanly with diamond grits and produces a high optical finish at 50,000 to 100,000 mesh polish.