Chemical composition -- Amorphous silica with a high water content.
Color -- Colorless, white, black, gray, red, blue, orange, yellow.
Optics -- R.I. 1.40-1.47.
Durability -- Hardness 5.5-6.5. Brittle and heat sensitive. May crack or craze spontaneously as the water content evaporates. Despite its frequent use in rings, opal is a poor choice for a ring stone and often is scratched or broken. It is much better suited to earrings, pins, and pendants.
Crystal structure -- Amorphous. The silica occurs as uniformly sized, tightly packed spheres. Light is reflected and refracted through this gridlike formation to produce the characteristic play of colors.
Specific Gravity -- 1.99-2.25.
Sources -- One of the most popular gems and found in many locations. Major suppliers include Australia, Brazil, Mexico, Czechoslovakia (formerly in Hungary), Nevada. Fine opals in large sizes are rare and costly, especially black opal. Usually cut as cabochons, sometimes beads. Occasionally found as fossilized (opalized) clamshells, snail shells, or wood. Transparent opals, such as Mexican red or orange fire opal, are often faceted. Values are normally determined by the presence and nature of color flashes (play of color).