Hardness and Toughness
Iolite rates between a 7 and 7.5 so it toughness is considered to be good, however, because it has perfect cleavage in one direction, iolite requires special care during cutting, polishing, and mounting. It’s not very tough, so a hard blow might split it, and extreme pressure or sharp temperature changes might cause it to break. To reduce the risk of breakage, jewelers prefer to set iolite gems in protective mountings or use them in pieces that aren’t exposed to too much wear, like pendants and earrings.
High heat or sudden temperature changes can cause breaks in iolite. The gem’s color is stable.
It’s important to avoid steam or ultrasound for cleaning iolite: Warm, soapy water works best.