Tiger's eye

Tiger's eye is quartz that has replaced an asbestiform amphibole — crocidolite, the blue asbestos — fibre by fibre, keeping the parallel fibrous structure while substituting silica for the original mineral. The iron in the crocidolite oxidises in the process, which is what gives the gold-brown colour.

The parallel fibres are why it does what it does. Light reflecting off a plane of aligned fibres produces a single moving band, which is called chatoyancy and is the same effect as a cat's eye. Cut as a cabochon with the fibres running across the dome, the band sweeps as the stone is turned.

Blue material in which the crocidolite has been replaced without oxidising is called hawk's eye, and the two occur in the same deposits, often in the same piece.

The asbestos question comes up and the answer is reassuring: the crocidolite has been replaced by quartz, so the finished stone is silica, not asbestos. Cutting any silica does generate respirable dust, which is a silicosis concern rather than an asbestos one, and lapidary work is done wet for that reason.

The chatoyant band is diagnostic and hard to mistake. Fibrous structure visible on a broken surface, quartz hardness, and a silky lustre. Dyed material in reds and blues is common and is usually obvious from the saturation.

Cabochons, beads, carvings and ornamental objects. Almost never faceted, because faceting destroys the one optical effect the material is bought for.

Descriptive, and translated more or less directly into most European languages. Most of the world's supply comes from the Northern Cape in South Africa.