Spectrolite
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Spectrolite is a hard and rare variety of labradorite feldspar. Spectrolite is between 6 and 6.5 hardness and weighs 2.69 - 2.70 g/cc. Spectrolite was discovered by the Finns during WWII when building a defense line. After the war, spectrolite was mined in small quantities. This small supply made it more valuable.