The name is a derivitave of the Indian word "stone". There are three groups of Opal: the opalescent precious Opals, special for their rainbow iridescence which changes with the angle of observation; the yellow fire opals, named after their orange color; and the common opal, which is mostly opaque and without play of color. It is most common. All opals have a measure of water in them ranging up to 30%. Opals can lose their water, thus dulling their color. The water can escape through cracks in the stone or evaporate from even a little heat. Their color can be temporarily restored by saturating them with water or oil. Opals are very brittle and are sensitive to knocks, pressure, heat acids, and alkalies.

• Color : White, gray, blue, green, orange
• Color of streak : White
• Mohs' hardness : 5 1/2 - 6 1/2
• Specific gravity : 1.98 - 2.20
• Cleavage : None
• Fracture : Conchoidal, splintery, brittle
• Crystal system : Amorphous kidney or grape shape aggregates
• Chemical composition : SiO2nH2O hydrous silicon dioxide
• Transparency : Transparent, opaque
• Refractive index : 1.44 - 1.46
• Double refraction : None
• Dispersion : None
• Pleochroism : None
• Absorption spectrum : Fire opal; 7000-6400, 5900-4000 Fluorescence: White: White Bluish, brownish, greenish
  Black: usually none
  Fire: greenish to brown