Opal

Those who love rainbows, often love Opals as well.  The play of color that they present, often referred to as "fire" within the stone, is quite reminiscent of the colors of the rainbow, although much more randomly arranged.  Only the electron microscope allows for the discovery of what causes this multi-color affect in the form of miniscule spheres of the mineral cristobalite layered within silicious jelly which causes the color diffractions. 

Opals will always have some amount of water in them, varying from 3% to 30%, which can, over time be reduced and thus diminish the fire within the opal. This issue can be temporarily be restored by saturation with epoxy resin, oil or water.  Opal is often combined with impregnated with plastic to improve appearance. Setting opals with traditional jewelry making techniques can be a little tricky as heat can evaporate the water within the stone and affect it's appearance.

Opals are often combined with other materials to enhance their appearance. The following are the types of combinations generally found.

Doublet:  Opal Doublets involve the adherence of a black backing such as black potch (colorless opal), black industrial glass, hard plastic or other materials.  The darkness of this undercoating significantly increases the color play of the thin opal layer that is then placed on top of it. 

Triplett:  Tripletts consist of three layers.  As noted above, a black backing is used, but an additional layer is laid on top of the opal which generally are comprised of a clear quartz, glass or plastic domed lens capping the top off.

Opals come in the following varieties:

White Opal:  Opals with a white or lighter basic color with color play.

Black Opal:  Opals with dark blue, dark green, dark gray or gray-black basic color with color play.

Opal Matrix:  Rock matrix with banded inclusions of opal.

Boulder Opal:  Found in pebble rock, where the opal has filled hollows.

Jelly Opal:  Bluish Gray opal with little color fire.

Crystal Opal:  These are transparent with good color fire on their surface.

Mexican Fire Opal:  Fire Opals usually show no color play, but derive their name from their brilliant orange coloring.

Deposits are obtained from Australia, Brazil, Guatamala, Hondurus, Japan, Russian, Mexico, Idaho and Nevada, U.S.A..

STATS:
Color:  All colors
Mohs' Hardness Scale: 5.5 to 6.5
Density:  1.98 to 2.50
Transparency:  Transparent, opaque