Topaz
A hard aluminium fluorosilicate spanning colourless to imperial orange — with a notable cleavage to respect.
Also known as: Aluminium fluorosilicate
beginner Granitic pegmatites and rhyolite
What it is
Topaz is aluminium fluorosilicate, Al₂SiO₄(F,OH)₂, and at 8 on the Mohs scale is one of the harder gemstones. Despite that hardness it has a perfect basal cleavage, so a sharp blow in the wrong direction can split a stone — cutters orient the table to avoid it. Pure topaz is colourless; impurities and treatment give blue, yellow, pink and the prized reddish-orange imperial topaz.
Most blue topaz on the market is colourless material irradiated and heated to a stable blue, a well-established treatment. Notable sources include Brazil (especially for imperial topaz), Pakistan, Nigeria and Russia. Large, clean crystals are common, so topaz offers big, bright stones at accessible prices.