Kirk Davison, age 12, of Burlingame, California, for his question:

What causes the flashes of light in an oral?

Opal is classed as a hydrous silicate mineral. Its silican molecules are made of silicon and oxygen atoms and opal may contain up to 10 per cent water in tiny pockets of moisture. These common chemicals glow with pearly opalescence because they reflect the penetrating rays of light. Opal is built in thin, glassy layers. They break up the wave lengths of light at various angles. The darting rays add the sparks and flashing to the lovely stone.

Opal may be translucent white, pale yellow or rich red. Precious opals flash more fiery colored sparkles than common opal, which is merely pale pearly white. All opal forms as desposits in mineral springs. Sometimes extra items get mixed with the basic materials. Trapped diatom shells and other impurities make the stone less hard and do nothing to improve its beauty.