Opal Glossary, technical descriptions on opals from Coober Pedy

OPAL GLOSSARY - Terminology descriptions on opals

Base Color - See BODY COLOR.

Body Color - The background tone of the stone, also called base color.

Black - Opal with a dark (black) body color: Rare.

Block Pattern - Pattern displaying large particles of color.

Boulder - Opal distinguished by the presence of host brown rock, usually ironstone, on the back of the opal seam; almost exclusively from Queensland.

Boulder Doublet - Usually cut from thin Opalized shells which are artificially attached to an ironstone backing.

Broad Flash - Large areas of flash in the opal.

Cabochon - Refers to the dome shape of the stone.

Calibrated - Opal cut to standard size, for example 7mm x 5mm oval.

Carat - Measure of weight used for gemstones (5 carats to a gram).

Chaff Pattern - Small blocks of color in opal giving an appearance of "chaff".

Chinese Hat - A nodule (knobby) of rough opal which is formed in the shape of a "Chinaman's hat".

Cobwebs - Thread-like impurity having the appearance of open weave knitting.

Copi - A white impurity in opal.

Color Bar - Layer of color in opal differing from color layers above or below.

Color Separation Line - Often confused with cracks by the inexperienced - this fine line may occur where different color formations meet.

Concrete - Very porous and soft matrix opal from Andamooka.

Cotton - Thread-like impurity in opal resembling cotton.

Crack - A fracture in opal which greatly, reduces the value.

Crazed - Network of cracks across an opal.

Crystal-Transparent or translucent opal "where" the colors are sharp and visible below the surface.

Diffraction - The breaking up of bean of a white :light causing spectral colors.

Doublet - A layer of light opal attached to a black background; simulates black opal.

Fan Harlequin - Color pattern of tiny squares in a tan shape.

Fire - Refers to the diffracted color play of an opal.

Fire Opal - A term used for that opal where the base color (as distinct from the diffracted color) has red or orange tone. This material usually comes from Mexico.

Flagstone - A pattern having defined patches of color with the appearance of flagstone paving. Less regular than harlequin.

Flaw - Mark or impurity which devalues the stone.

Freeshape - An opal cut into an irregular shape or not a calibrated size.

Freesize - An opal not cut to a standard calibrated size.

Flash - A pattern which consists of large flashing areas of color. Also see Broad Flash and Rolling Flash.

Fun Stone - Boulder opal with prolific ironstone on the surface.

Gem Quality - Finest grade.

Ginger Whiskers - Rust colored marks in opal often associated with cracks.

Gypsum - Common impurity in opal, a needle-like crystal of this may go right through the stone.

Harlequin - Very rare and most sought after opal, the colors form naturally in checkerboard pattern - this is a collector's item.

Impurity - Foreign particle in the stone such as sand spot or gypsum.

Jelly - Transparent opal with "jelly" appearance, colors generally hazy.

Light - Refers to the body or base colors.

Mackerel Sky - See Ribbon

Matrix -
(1) Porous opal from Andamooka often dyed dark to simulate black opal.
(2) Ironstone containing veins of precious opal which have formed in rivulets. (Exclusive to Queensland)
(3) Miners at Lightning Ridge use this term to describe a porous white impurity in black opal.

Milky - An opal with a creamy body color which dominates the diffracted color.

Multi-Colored - A stone that has more than 3 colors in it, but may have one color dominating.

Multi-Fire - A stone which has lots of colors without any particular color dominating.

Nobby - A formation of rough opal occurring in Lightning Ridge - a round nodule.

Painted Lady - Exclusive to Andamooka - a thin layer of opal on hard sandstone, usually sold as specimens.

Pineapple - Very rare collector's item - exclusive to the White Hills opal field in south Australia. A cluster of Opalized crystals which are a pseudomorph of ikiate crystals.

Pinfire - A pattern of tiny dots of color across the stone.

Pipe Opal - (1) In Queensland, this refers to tubular-like opal which forms when fossil wood becomes Opalized.
(2) In Coober Pedy, cigar shaped opal which forms when opal replaces a fossil squids backbone (belemnites).

Potch - Common non-precious opal without diffracted colors.

Red on Black - Type of black opal which displays predominantly red. Extremely valuable.

Refraction - The deflection of a ray of light when passing obliquely out of a substance.

Ribbon - Pattern where colors roll in parallel bands across the stone.

Rolling Flash - Unusual pattern displaying colors which "roll" across the face of the stone with movement.

Rough - Uncut opal, as we find it, but cleaned.

Seam Opal - Opal which occurs in horizontal seams; the most common type of formation: can be found in all fields.

Semi Black - This opal has dark background but is not dark enough to be termed "black".

Skin to Skin - A opal stone that has the color from the top to the bottom or all the way though, as all seam opal has a skin on the top and bottom of it from when it was formed millions of years ago, or in the case of Verticals this skin is on the sides, so if the opal is full of color between these two skins, we call it skin to skin here in Coober Pedy.

Solid - An opal which has not been backed, capped or treated by man.

Straw Pattern - Rare pattern - the colors in the opal look like straws.

Translucent - See Crystal.

Triplet (Triplex) - A thin slice of light opal attached to a black or dark background with a protective crystal or glass dome glued to the top.

Vertical Opal - This opal appears to be banded and forms vertically in the earth rather than horizontally like seam opal.

White Opal - The body color has a white appearance.

Yowah Nut - Found in Yowah, Queensland. The opal forms as a kernel inside an ironstone concretion.