OPAL

Extremely fine quality Australian black opal necklace

Species: Opal
Mohs Hardness: 5.5 to 6.5 out of 10
Color: Colorless, white, black, gray, bluish gray or brown. With or without play-of-color. Fire opals are orange or orangy red, with or without play-of-color.
RI: 1.450 (+0.020 / -0.080)
Birefringence: None
SR/DR/AGG: SR, commonly with ADR due to strain
Spectrum: Not diagnostic
Fluorescence: Inert to white to moderate light blue, green, or yellow under LW and SW. Might phosphoresce.
SG: 2.15 (+0.08 / -0.90)
Routine Treatments: None
Additional Enhancements: Dyeing, smoke treatment, sugar treatment

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opal facts

1950s vintage J.E. Caldwell platinum ring featuring a GIA-certified 10.07 carat untreated black opal, accented by sapphires & diamonds

1950s vintage black opal and sapphire ring, by J.E. Caldwell

Opals contain up to 20% water trapped inside their silica structure.

Rains would settle into the cracks in Australia's semi-desert Outback, soaking into the ancient underground rock. After the water evaporated, it would leave silica deposits which would form opal.

Opals are known for their unique phenomena called "play-of-color". Opals are made up of sub-microscopic spheres stacked in a grid-like pattern. As light passes between the spheres, the lightwaves bend, breaking up into different spectral colors.

Not all opals display play-of-color.

There are several different types of opals: white opals, black opals, crystal opals, boulder opals and fire opals.

Fire opals are a transparent yellow to orange, to orangy red, often faceted, and can be with or without play-of-color.

Opal is an October birthstone.

opal gallery

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play-of-color patterns

pinfire

Photo courtesy of Scott Papper, GIA GG, AJP, CM (NAJA), Global Gemology & Appraisals

Small, closely-set patches or dots

flame

Flash play-of-color pattern in this precious white opal

Photo courtesy of Scott Papper, GIA GG, AJP, CM (NAJA), Global Gemology & Appraisals

Large areas that flash predominantly one color, usually red or orange, as the gem is moved

harlequin

Photo courtesy of Scott Papper, GIA GG, AJP, CM (NAJA), Global Gemology & Appraisals

Broad, angular, closely set patches of color that can resemble brush strokes

opal treatments

sugar treatment

Photo courtesy of Opal Auctions

Usually has a lower RI than untreated black opal (1.44 to 1.45 RI). Black carbon particles visible in and around gem opal, as well as in the surrounding matrix. Appears in a mosaic pattern and will show white material underneath the surface at chipped areas.

smoke treatment

1.38 to 1.39 RI. Has a dark brown, mottled appearance with unnatural play-of-color. Turns black when wet, but blackness disappears as the stone dries.

opal imitations

synthetic opal

Imitation opal has a honeycomb-like, structured appearance

Photo courtesy of synthetic-opals.com

Use magnification to observe a "chicken wire", "snakeskin", or scale-like structure. May show columnar structure when viewed from the side.

doublets / triplets

Opal triplets may look very convincing from the top, but a profile view reveals their assembled structure. [Photo courtesy Scott Papper, GIA GG, AJP, CMA (NAJA)]

Opal doublets use a very thin piece of opal cemented to clear top.

Opal triplets use a thin piece of opal sandwiched between a dark backing (usually black onyx) and a clear top (often clear quartz or glass).