Opal Formation and Mining

Where Does Opal Come From? Opal is found in Brazil, Honduras, Indonesia, Mexico and western parts of the USA, but it is the vast deserts of central Australia which produce 95 percent of the world's suply of precious opal. Somewhere between 65-140 million years ago in what is now termed the Cretaceous period, these desert areas were an inland sea. The sea gradually receded but not before sands rich in silica had been deposited on the shoreline. About 30 million years ago,. during the mid-Tertiary period, climatic changes caused quantities of soluble silica to be released from the sediment. This gel-like solution filtered along cracks and faults in the ground, filling them, and eventually hardening to form common opal and, more rarely, precious gem quality opal. What Kind of Mineral Is Opal? As we have already established, opal is a silica gel. It consists of tiny almost identical transparent spheres of silica arranged together in a uniform three dimensional grid. Silica in solution is held in the spaces between these solid silica particles. The difference between common opal and porecious opal is that the silica spheres in common opal are different sizes and not arranged in an orderly manner. The Rainbow Colour Light passes directly through the spheres but is deflected off the silica solution. Depending on the size of the spheres, this diffracted light can show a handful of dominant colours or all the colours of the spectrum. Smaller spheres, for example, display blue colours while larger spheres show orange and red.