Opal

Opal was a milky gem stone related to black and fire opals, but occurred slightly more frequently.

Description
The substrate of opal was usually an opaque pale blue, while the inclusions flashed green to gold. Opals were usually cut cabochon or used as inlays, and a typical specimen had a base value of 1,000 gp.

Powers
Opals had a high affinity to magic and were used in casting numerous spells and in the creation of magic items, especially those that stored spells for later use. The most well-known example was the helm of brilliance. Opals could be substituted for other, more scarce materials when constructing such items because of its ability to release spells just as if they were cast normally, with no change in range, power, or effect. When enchanting opal or using it as part of a magical procedure, the best results were obtained at twilight.

One extremely important caveat regarding opal: it was never to be used in creating items that attempted to combine fire and lightning. Doing so resulted in an immediate chain reaction/explosion in which a large bolt of chain lightning was released with the additional effect of two Melf's minute meteors released in random directions whenever a cascading target was hit.