Topaz: a hard stone with one real weakness

Topaz sits at 8 on the Mohs scale, hard enough that it is a standard reference point on the scale itself, but it has perfect basal cleavage, meaning it can split cleanly along one direction if struck at the wrong angle despite its hardness. This is a genuinely important practical fact for anyone cutting or even just handling rough topaz, hardness protects against scratching but does nothing to protect against a clean cleavage break from an impact. Natural topaz colors include colorless, pale blue, and golden brown to sherry colored tones, the intense blue seen in a lot of commercial jewelry is very commonly the result of irradiation and heat treatment of naturally pale or colorless material, a widely disclosed and standard industry practice.