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block pattern
chinese writing
jelly opal
flagstone opal
rolling flash pattern

Types & Patterns of Gem Opal

The descriptions below are taken, by kind permission, from Andrew Cody's book Australian Precious Opal (R1481). The reader should refer to that publication for an authoritative account of Australia's national gemstone:

Block pattern: pattern displaying large particles of colour
Broad flash: large areas of flash in the opal
Chaff pattern: small blocks of colour giving an appearance of chaff
Chinese writing: patches of colour forming hieroglyphs reminiscent of Chinese script
Crystal: transparent or translucent opal where the colours are sharp and visible below the surface
Fire opal: a term used for that opal where the base colour (as distinct from the diffraction colours) has a red or orange tone. This material generally comes from Mexico
Flagstone: a pattern having definite patches of colour with the appearance of flagstone paving. Less regular than harlequin
Flash: a pattern which consists of large flashing areas of colour
Harlequin: very rare and most sought after opal, the colours form naturally in a chequerboard pattern. This is a collector's item footnote
Jelly: transparent opal with 'jelly' appearance, colours generally hazy
Milky: an opal with a creamy body colour which dominates the diffracted colour
Multi-fire: a stone which has most colours without any particular colour dominating.
Pinfire: a pattern of tiny dots of colour across the stone.
Red on black: type of black opal which displays predominantly red. Extremely valuable.
Ribbon pattern: colour bands across the stone.
Rolling flash: unusual pattern displaying colours which 'roll' across the stone.
Straw pattern: rare pattern; the colours in the opal look like straws.