Natural Gemstones - Loose Precious & Semi Precious Gemstones

    * 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.