Opal in matrix

Turn a precious opal this way and that, and watch the colors dance.

Within the opal, tiny spheres of silica--stacked up like oranges--scatter light, causing the play of colors. Opals with this fiery flash are prized as gems. But water comprises 6-10 percent of their weight, and heat can dry them out and make them crack. Opal is the October birthstone.

Take a look at:

• black opals, in which colors flash against a dark background.
• white opals, in which colors play against a white background.
• opals with no play of colors.