Hawk's Eye (Blue Tiger's Eye)

Hardness: 6.5-7 on Mohs scale; Color: Dark blue-gray to blue-green with iridescent bands; Luster: Silky, vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.64-2.71

Hawk's Eye forms when fibrous crocidolite asbestos (a blue amphibole mineral) is replaced by quartz over time, a process called pseudomorphism. The fine, parallel fibers of the original crocidolite are preserved, creating the characteristic chatoyancy (cat's eye effect) when polished. The blue color is due to the presence of unaltered crocidolite fibers. Geologically, it forms in metamorphic environments, often associated with banded iron formations, and can be millions to billions of years old.

Primarily used as a gemstone in various forms of jewelry, including beads, cabochons, and carvings. It is also collected by mineral enthusiasts.

Hawk's Eye is a precursor to Tiger's Eye; if the iron content of the crocidolite oxidizes (rusts) during the silicification process, the mineral turns golden-brown, becoming Tiger's Eye. The chatoyancy in Hawk's Eye is often more subtle and less pronounced than in golden Tiger's Eye, appearing as a shimmering band of light that moves across the surface. It is often believed to bring protection and good fortune.

In the field, Hawk's Eye is identified by its distinctive blue-gray to blue-green color, fibrous appearance, and the characteristic chatoyancy when polished or rotated under light. It's commonly found in South Africa (Griqualand West), Western Australia, India, and the USA (California). For collectors, look for strong chatoyancy and an even color distribution. Avoid pieces with significant pits or cracks, as these can affect durability and appearance.