Opal

Opal 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).

Opal's scratch hardness is measured at 6.0 to 6.5 on the Mohs' scale, similar in hardness to quartz, a little more than halfway between the hardness of talc and diamond.