Hawk's Eye

Hawk's eye occurs in the same banded-iron-formation deposits as yellow tiger's eye, where the fibres stayed less oxidised. Near Prieska in South Africa's Northern Cape, Mintek geologists recorded blue tiger's eye (silicified crocidolite), yellow and variegated tiger's eye; blue and variegated material made up about 18% of the high-grade zones, about 50% of the low-grade zones and about 60% of the fibrous zones. Pockets of unsilicified crocidolite asbestos next to the seams are an asbestos hazard for miners. Mindat shows photographed specimens from the Northern Cape, South Africa, and from Kryvyi Rih, Ukraine.

Turn the stone to see a silky band of light move across a dark blue, opaque, fibrous body. Partly altered pieces show blue and golden zones side by side (variegated tiger's eye). GIA describes hawk's eye as dark blue; brightly coloured blue tiger's eye may be dyed, and IGS illustrates dyed blue cabochons. GIA notes that a qualified gemologist can detect dyed gems in most cases, and that chatoyant glass and plastic made from fused optical fibres are cheap imitations of cat's-eye stones.

IGS rates the wearability of tiger's eye, including hawk's eye, as very good; it has a hardness of about 7 and no cleavage.