Opal is one of the most popular gemstones worldwide, however, unlike most stones, it is not a true mineral, and is classified as a mineraloid. This is because opal lacks a crystalline structure like most other gemstones. Rather opal is formed when water running through the bedrock lifts and carries nanometers-small particles of silica and deposits them into the nearest crack or void. Over millions of years this builds up very fine, tightly packed layers of silica, with regularity down to the atomic level. This creates the opal we’re familiar with, the beautiful optical properties being a result of light bouncing and reflecting between the microscopic layers.
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photo:wikipedia