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The multicolored flashes of light that Opal

Opal has been a popular gem for many centuries and has a very interesting structure. Opal is considered a mineraloid because this structure is not truly crystalline. The chemistry of Opal is primarily SiO2 and varying amounts of water. The amount of water varies from 5 -10% and greater. This water can help geologists determine the temperature of the host rock at the time the opal formed.

Although there is no crystal structure, (meaning a regular arrangement of atoms) Opal does possess a structure nonetheless. Random chains of silicon and oxygen are packed into extraordinarily tiny spheres. These spheres in most Opals are irregular in size and inconsistent in concentration. Yet in Precious Opal, the variety used most often in jewelry, there are many organized pockets of the spheres. These pockets contain spheres of approximately equal size and have a regular concentration, or structure, of the spheres. This has the effect of diffracting light at various wavelengths, creating colors. Each pocket produces a different color, with a different intensity depending on the angle from which a viewer sees it. The multicolored flashes of light that Opal emits gives it a truly beautiful and valuable look.

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  • 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: Crystals 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 or as a replacement of other minerals and wood.
  • Cleavage is absent.
  • Fracture is conchoidal.
  • Hardness is 5 - 6
  • Specific Gravity is approximately 2 - 2.5 (light)
  • Streak is white.
  • Associated rocks are chert (a form of microcrystalline quartz), volcanic rocks and many others.
  • Other Characteristics: Some opal fluoresces and it can be very sensitive to impacts and low temperatures.
  • Notable Occurrences include Western USA; Mexico; Australia and many other localities around the world.
  • Best Field Indicators are color play (opalescent), low density, lack of cleavage and crystal faces and fracture filling tendency.

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