Precious opal, which displays 'opalescence' (spectral color, irridescence that c hanges with the angle at which the gem is viewed).

Two subgroups:

White opal is an opaque stone in which the colors appear as flashes or speckles.

Black opal contains fire with a dark body color. These are less common and tend to be costly.

Examples:

• precious opal note the rainbow-like iridescence.
• colors and patterns
• opalized wood
• shell
• stem

Fire opal is transparent or translucent with an orange or red body color. Fire opals are named for their color (but are not opalescent). The term is often misinterpreted to indicate that fire opal is opalescent because in precious opal, (with a play of color) the play itself is called fire.

Examples of fire opal and related varietites

(Mexican) water opal is transparent a nd contains flashes of fire.

• jelly opal
• also found in a rhyolite matrix
• "water-bearing opal"

  Water can still be seen inside the opal!

• fire opal
• note the orange color, no opalescence

Common opal is rather opaque. Many names are used to describe varieties, e.g., honey opal, milk opal, moss opal, etc.

Examples:

• common opal: opaque, no play of color
• "petrified" opal

• often as linings of cavities or crack fillings

Geographical: 90 % of World's supply of opal from South Australia

Doublets and Triplets:

Doublets and triplets consist of a thin slice of opal cemented to a dark substrate. In the case of opal triplets, a colorless glass or crystalline cap is cemented to the opal slice.

Doublets and Triplets are common, as these are substantially less expensive than precious opal.

Examples:

• doublets
• triplets.

1. impregnation with plastic, surface oiling
2. carbon or sugar treatment:
   • heating the opal in paper (manure) leaves a deposit of carbon below the surface of the stone
   • soaking the opal in sugar-rich solutions in sulfuric acid bath gives it a peppery 'pinfire' effect (play of color)
     (this is a superficial treatment, which leads to low S.G. and porus stones)

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   SYNTHETIC OPAL

   • Several manufacturers, including Gilson, Opal, Inamori Opal, etc.

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   SIMULANTS

   • glass: "slocum stone"
   • foil-backed/reflected light: foil-like particles and cellophane-like foils of transmitted light
   • may contain obvious bubbles

   

   View some opal images!

   

   Precious Stones: Lapis, Turquoise, Malachite and Azurite

   Pearls

   

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