Opal

Formula: SiO₂.nH₂O

Origin of name: from Sanskrit upala = precious stone

Crystal system: amorphous

Crystal class: 

Twinning: 

Unit cell: 

Colour: colourless, white, bluish, yellow, red, brown, orange

Diaphaneity: 

Luster: vitreous, resinous, pearly, waxy

Habit: massive, compact, porous or earthy, rock-forming, veinlets, cavity fillings, globular, botryoidal, stalactitic

Hardness: 5.5 to 6.5

Specific gravity: 1.99 to 2.25

Cleavage: none

Tenacity: 

Streak: white

Synonyms/varieties: 

Precious Opal (with play-of-color)

black opal: translucent to opaque with dark body-colour
boulder opal: occurring in thin seams
crystal opal: colourless, transparent 
fire opal: transparent with red/orange body-colour
harlequin opal: broad, angular patches of colour
white opal: translucent with white body-colour

Common Opal (without play-of-color)

fire opal: transparent with red/orange body-color
girasol opal: translucent, resembling moonstone
hyalite/jelly opal/water opal: colourless, transparent
potch opal: translucent to opaque
prase opal: translucent to opaque, green

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