The chief mineral constituent in the ornamental stone lapis lazuli. It crystallizes in the isometric system, but well-formed crystals, usually dodecahedral, are rare. Most commonly, it is granular or in compact masses. There is imperfect dodecahedral cleavage. The hardness is 5–5.5 on Mohs scale, and the specific gravity is 2.4–2.5. There is vitreous luster and the color is a deep azure, more rarely a greenish-blue. Lazurite is a tectosilicate, the composition of which is expressed by the formula Na₄Al₃Si₃O₁₂S, but some S may be replaced by SO₄ or Cl. Lazurite is soluble in HCl with the evolution of hydrogen sulfide.