Kyanite, an aluminium silicate, is an extremely attractive gemstone deriving its name from the Greek kyanos, meaning 'blue' (although it can also be colourless, white, gray, green and yellow), and the best material has a clear, royal blue colour. It is unique in having two hardness values which makes it difficult to cut. Not normally treated. Hardness: 4.5 and 7 (Mohs).

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