Cordierite

Cordierite: a Magnesium, Iron, and Aluminum Silicate with the formula (Mg,Fe)₂Al₃(AlSi₅O₁₈). It crystallizes in pseudo-hexagonal prisms (Orthorhombic System); violet-blue, grayish-blue, rarely greenish or yellow-brown in color. Specific gravity: 2.53–2.78 g/cm³; Mohs hardness: 7–7.5; refractive indices: 1.522–1.558; 1.524–1.574; 1.527–1.578.

(Photo: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0/Wikimedia Commons, License CC BY-SA 3.0)

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