The Complete Opal Guide

Because opals have varying degrees of water content, they are delicate – especially when subjected to heat, temperature changes, changes in air pressure (such as in an airplane)
Dampened soft fabrics with no abrasive or chemical additives, or a soft bristle toothbrush doused with water

Play-of-Color: impregnation with oils, wax or plastic
Color: Dyeing or smoke impregnation: this treatment causes lighter opals to look like darker, black opals, which are considered more valuable.
Color: Reflective foil-backing: this treatment darkens the gem and improves play-of-colour. Not easy to detect when opal is in jewelry
Color: Black paint backing: see foil backing
Several other treatments to stabilize the gem

Many colours are seen in opal. Body colours can vary from white to dark blue and to black, with brown, red orange in between.
In recent years, a turquoise blue and a pink opal variety, owing their colour to traces of copper, has been discovered in Peru
More recently the opal find from Welo, Nigeria show an extensive color play in “harlequin shapes”
Most commecial types: white, black, fire opal

Collectors prize one-piece opals (without matrix or backing) that display strong play-of-colour. Collectors look for patterns such as “harlequin,” which shows a broad flash of colours when the gem or light source is moved; “pinfire,” which exhibits tiny flashes of multi-colour patches. White opals can also show these characteristics. Contra-luz opals are also collected because of their relative rarity, and their dramatic reactions to light.

Opals are rarely faceted because the facet edges and junctions are prone to abrasion
Some Mexican, Peruvian and crystal opals are faceted and these tend to exhibit a sleepy, milky appearance on colourless or coloured bodycolour
Most are cut en cabochon, which avoids abrasion along stark edges and are an appropriate canvas upon which to best exhibit an opal’s play-of-colour

Translucent, semi-translucent, opaque