* 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.
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