Beryl

Formula:  Be₃Al₂Si₆O₁₈

Origin of name:  From Greek beryllos - a blue-green gem

Crystal system:  Hexagonal

Crystal class:  6/m 2/m 2/m

Twinning: Rare, on {hk-il}

Unit cell: a = 9.215 Ǻ, c = 9.192 Ǻ

Colour:  Colourless, white, light green to green, yellow, pink to pinkish orange

Diaphaneity: Transparent to translucent

Luster:  Vitreous

Habit:  Crystals short to long prismatic; often etched

Hardness:  7.5 to 8

Specific gravity:  2.6 to 2.9

Cleavage:  1; {0001} indistinct

Tenacity:  Brittle

Streak:  White

Synonyms/varieties:  Aquamerine (green-blue variety), emerald (green variety), goshenite (colourless variety), heliodor (yellow variety), morganite (pink variety), bixbite (red variety)

Comments: Found in granites and granite pegmatites, rarely in nepheline syenites. Also some metamorphic rocks, low- to high-temperature hydrothermal veins and vugs in rhyolite