Lazurite

Formula:  (Na,Ca)_7-8(Al,Si)₁₂(O,S)₂₄[(SO₄),Cl₂,(OH)₂]

Origin of name:  From Persian, ;azhward = blue

Crystal system:  Cubic

Crystal class:  -43m

Twinning: 

Unit cell: a = 90.9 Ǻ

Colour:  Deep blue, azure blue, violet blue, greenish blue

Diaphaneity: 

Luster:  Dull

Habit:  Crystals dodecahedrons, rare; massive, compact

Hardness:  5 to 5.5

Specific gravity:  2.38 to 2.45

Cleavage:  1; {110} imperfect

Tenacity:  Uneven; brittle

Streak:  Bright blue

Synonyms/varieties:  Cyanus, lapis lazuli, ultramarine

Comments:  May also be monoclinic, triclinic or orthorhomibic