PRECIOUS OPALS
The special characteristic of these gems is their opalescence, a rainbow-like iridescence which changes with the angle of observation. Until the 1960s, this was thought to be caused by the refraction of light from the thin surface layers. The real cause was discovered under the electron microscope using a magnification of 20000: tiny spheres (o.oo1 of a millimeter in diameter ) of the mineral cristobalite layered in siliceous jelly cause the reflection or interference appearances. Strictly speaking, precious opal is not really amorphous.
Opal always contains water; the content varies but it can be as much as 30%. It can happen that in the course of the time, the stone loses water, cracks and the opalescence diminishes. This can, at least temporarily, be restored by saturation with oil or water. The aging process is avoided and the opalescence increased when stored in moist absorbent cotton ( cotton wool). Care must be taken during setting. A little heat can evaporate the water. opal is also sensitive to pressure and knocks as well as being affected by acids and alkalies.
Two groups of opals can be distinguished: those with a white or light basic color, known as white or milky opals, and the rarer black opals. Their color is dark gray, dark blue, dark green or gray black. Deep black is rare.
Opal matrix (formerly known as opaline) is a blended or leafed inclusion of precious opal in the matrix rock. Cut as gemstones because of the good content with associated ,materials.
In Europe precious opal has been thought of as unlucky, but in the orient it stands for loyalty and hope. The play color is seen best if it is cut en cabochon. Very thin pieces of opal are sometimes mounted on a piece of common opal or onyx; this the opal doublet. Triplets are also made with a protective top layer of rock crystal.

*Info by Walter Schumann “Gemstones of the World”