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 black opal and sapphire ring, by J.E. Caldwell

Opal is a unique gem known for its unique phenomenon called "play-of-color".  However, not all opals display 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.  Opals can contain up to 20% water!


There are several different types of opals:

• White Opal
• Black Opal
• Crystal Opal
• Boulder Opal
• Fire Opal


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

In 1938, the Aurora Australis opal was dug up from an old sea bed at Lightning Ridge, New South Wales and is considered to be the most valuable Australian black opal.  In 2005, it was valued at AUD$1,000,000.  Aside from its vibrant blues, greens, and reds, it also has a distinctive impression of an actual starfish on its reverse!

The Fire Queen is a famous opal with an unusual history attached to it.  Originally named The Dunstan Stone after its finder, Charlie Dunstan, this opal weighed over 900 carats and was deemed "too beautiful for words".  At the time, Dunstan wound up selling the opal for a mere £100.  After losing two more stones of value, Dunstan was found dead as a result of suicide, in 1910.  In 1928, after changing hands several times, the opal wound up in the Chicago Museum.  In the 1940s, J.D. Rockefeller purchased the opal for £75,000, adding it to his personal collection.

Opal Gallery

Play-of-Color Patterns

PINFIRE

​Small, closely-set patches or dots

FLAME

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

HARLEQUIN

​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

LAB CREATED OPAL

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.

​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).

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