hawk’s-eye, noun

Mineralogy. A blue or greenish variety of tiger’s eye, a mineral formed by the natural alteration of crocidolite (blue asbestos) when the asbestos fibres are replaced by quartz; the semi-precious gemstone made from this mineral. Cf. doeksteen.
1946 L.J. Spencer Key to Precious Stones 199Associated with it (sc. tiger’s-eye) is found ‘hawk’s-eye’ of a dark blue colour; here the blue crocidolite was silicified without previous oxidation and decomposition.
1971 D.J.L. Visser in Std Encycl. of Sn Afr. III. 503The fibrous colour depends on the degree of oxidation and hydration of the iron...Colours vary from red (bull’s-eye), golden brown (tiger’s-eye), greyish yellow to blue (cat’s-eye) and ash-greyish green (hawk’s-eye).
1977 J.E. Arem Color Encycl. of Gemstones 99Crocidolite (blue asbestos) may decompose and alter to quartz, retaining the fibrous structure. This can be further cemented by quartz and stained by iron oxides to yield a dense, siliceous, fibrous material called tigereye...If the material is unstained by iron and therefore solid blue, it is called hawks-eye. Occurrence: South Africa.
1980 Bates & Jackson Gloss. of GeologyHawk’s-eye,..a blue variety of tiger’s-eye.
A blue or greenish variety of tiger’s eye, a mineral formed by the natural alteration of crocidolite (blue asbestos) when the asbestos fibres are replaced by quartz; the semi-precious gemstone made from this mineral.

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