SiO2 - nH2O; Hydrated Silicon Dioxide.
# Color is white, colorless, pale yellow, pale red, gray or black when impurities are common. Diffraction can cause flashes of any color of the rainbow (opalescent).
# Luster is vitreous to pearly.
# Transparency: Specimens are transparent to translucent.
# Crystal System: Does not apply because opal is amorphous.
# Habits include massive, cavity-fillings such as in fractures and geodes, nodular, reniform or as a replacement of other minerals and wood.
# Cleavage is absent.
# Fracture is conchoidal.
# Hardness is 5.5 - 6
# Specific Gravity is approximately 2 - 2.5 (light)
# Streak is white.
# Other Characteristics: Most specimens will fluoresce white or pale green, some phosphoresce and all specimens can be very sensitive to impacts and low temperatures.
# Associated rocks are chert (a form of microcrystalline quartz), volcanic rocks and many others.
# Notable Occurrences include many Western USA localities; Mexico; Australia; England; Czech Republic and many other localities around the world.
# Best Field Indicators are color play (opalescence), low density, fluorescence, fracture filling tendency and lack of cleavage or crystal faces.