A sodium iron amphibole, often dark blue to black. Crocidolite (blue asbestos) is a fibrous variety, which when silicified forms Tiger's Eye. It's associated with insight, problem-solving, and calmness. (Fibrous forms are asbestiform, handle with care).
Dark Blue, Black, Greenish-Black, (Crocidolite: blue, fibrous)
Riebeckite is classified as a silicate mineral mineral which typically crystallizes in the monoclinic system. Its chemical formula is Na2(Fe2+3Fe3+2)Si8O22(OH)2. Common crystal habits include as prismatic crystals, commonly fibrous, asbestiform; earthy, massive.