![]() ![]() Opals have a very long history with the human timeline & have been revered for thousands of years. As this Manganese rich solution flows through it creates unique dendrite crystals which vary in shape & size depending on the concentration ratio of Opal to mineral solution of the various manganese oxides & hydroxides involved. Depending on the conditions in which it formed & the alignment & size of the silica spheres it can take on many colours.ĭendritic Opals are unique in the Opal family they form when water rich percolating mineral solutions containing manganese & iron flows along naturally occurring fractures, fissures & bedding planes. These ordered silica spheres produce the internal colours by causing the interference & diffraction of light passing through the microstructure. At micro scales it is composed of silica spheres some 150 to 300nm in diameter arranged in a hexagonal or cubic close-packed lattice. Although Opal is a mineraloid, it has an internal structure. It is most commonly found with limonite, sandstone, rhyolite, marl & basalt. It forms at a relatively low temperature by descending percolation of surface mineral solutions & may occur in the fissures of almost any kind of rock. Because of its amorphous character it is classed as a mineraloid, unlike the other crystalline forms of silica which are classed as minerals. Opal is a hydrated amorphous form of silica its water content may range from 3% to 21% by weight, but is usually between 6% & 10%. ![]()
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