Lapis Lazuli

Lapis Lazuli: it is not a mineral but a rock, composed mainly of a complex aggregate of Lazurite, Sodalite, Haüyna, Noseana, Calcite and Pyrite. Chemical composition of Lazurite, according to various authors: Na₄Si₂Al₃Si₃O₁₂ (Dictionary of Gems & Gemology, GIA); Na₇Ca(Al₆Si₆O₂₄)(SO₄)(S₃)• H₂O (Mindat); Na₃Ca(Al₃Si₃O₁₂)S (Webmineral); (Na,Ca)_7-8(Al,Si)₁₂(O,S)₂₄[(SO₄),Cl₂,(OH)₂] (Gems, R. Webster). Crystal system: cubic, gem material occurs as a granular crystalline aggregate. Luster: glassy to greasy. Transparency: opaque. Color: blue, purple, greenish, with white, gray and metallic patches. Mohs hardness: 5–6. Density: from 2.50 to 3.00 g/cm³. Refractive Index n = 1.50.

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