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Gemstone Crystal System Density Hardness Refractive index Treatments
amorphous 2.15 5.0-6.5 1.450 fracture filling

Opals, rough and cut

Colorless, white, black, orange and yellow.

Moderate, wear with care

Australia, Brazil, United States, and other locations world wide

Archaeologist Louis Leakey found six-thousand year old opal artifacts in a cave in Kenya!


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Opal Grading and Classification

In 2000, The Australian Gemstone Industry Council established nomenclature and classification standards for all types and origins of opals, This classification has been adopted internationally.
Opals can be broken down into 2 basic categories:

Precious Opal : any opal displaying play-of-color. This phenomenon is caused by the diffraction of white light thru a microscopic, orderly arrangement of silica spheres. This category includes white, black and boulder opal. Precious opal can be further distinguished by types:

Type 1: A single, solid piece of precious opal, having a uniform appearance and composition. This is the type of opal most commonly used for jewelry
Type 2:Precious opal that is attached to its host rock (a non-opal) in the form of a layer or seam. Boulder opal is an example of this. The opal is attached to a brown, iron-stained sandstone.
Type 3: Matrix opal occurs when precious opal fills cracks and openings in the host rock. The opal forms in pre-existing clay or sandstone. This material is frequently dyed.
Doublets & Triplets: These assembled stones are not considered natural opals, although they do contain a layer of natural opal.

Common Opal or Potch : These are varieties of opal that do not show a play-of-color. Although they share the same chemical composition as precious opal, the silica spheres they contain are randomly arranged.



Judge the body color of an opal "face up".
If an opal has very dark potch on the back, giving it the appearance of N5, it should be graded as N5.


Opals can vary in degrees of transparency from transparent to opaque. When an opal is transparent or semi-transparent it is referred to "crystal". This is true regardless of the body tone. "Crystal" refers to the glass-like appearance of the gem, NOT a crystalline structure.


  • Treatment with aniline dye, silver nitrate or sugar carbonized with acid.
  • Impregnation with oil, wax, or plastic.
  • Smoke impregnation.
  • Impregnation with black plastic.
  • Backing with foil, black paint or laquer.



Opal is a hydrous, silicon dioxide. It is unlike other minerals because it is not crystalline! It is considered to be a hardened jelly. It's water content varies from about 2 to 21% mineralogically, but about 6 to 10% in gem opals. Gem opals are those with a play of color and/or transparent orange varieties. The varieties of gem opal are as follows:

White Opal Light or white body color with play of color
Black Opal Black, gray, dark blue or green body color with play of color
Fire Opal Transparent or translucent orange or red, with or without play of color
Water Opal Colorless, transparent variety with play of color

The play of color, or fire can be described as follows:
Pinpoint/Pinfire Numerous small points of fire
Flash Broad flashes of color that disappear when the stone is moved
Harlequin Large angular patterns,like the diamonds in a harlequin patters

Opals have a rather bad reputation, stemming from the plot of a popular Sir Walter Scott novel in the 19th century. The heroine of the novel has her life force caught in the beautiful opal she wears in her hair and she dies when the fire in the opal is extinguished. Opal was also treasured in the Middle Ages and was called ophthalmios, or eye stone, due to a widespread belief that it was beneficial to eyesight. Blonde women wore opal necklaces to protect their hair from losing its color. Some thought the opal's effect on sight could render the wearer invisible. They were popular talimans for theives. According to aboriginal legend, opals are devils, half-serpent, half-human, who lurk in holes in the ground, luring men to their destruction with flashes of magic. All that being said, I must add, that I have owned, worn, cut and collected opals since I was a teenager, with no apparent ill effects.

Ethiopian Opal:

Ethiopian Opal