"Tiger´s eye (also called tiger eye) is achatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre. As ...
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members of the quartz group, tiger´s eye and the related blue-coloured mineral hawk´s eye gain their silky, lustrous appearance from the parallel intergrowth of quartz crystals and altered amphibole fibres that have mostly turned into limonite." - (en.wikipedia.org 11.12.2019)
