Most people are more familiar with australian opal than the mexican variety. Opals consist of veins of silica which are found in the host matrix rock. In the case of australian opal veins of opal are often found in ironstone which may be polished along with the opal to produce boulder or koroit opal. Mexican cantera opal, which is the equivalent of boulder opal, is found in pale rhyolite matrix which is softer than the opal itself. The play of colour seen in all types of opal depends on the arrangements of the microscopic spheres of opal silica which diffract the incident light. If the arrangement of these spheres is haphazard there is no play of colour and the opal is referred to as potch opal. A regular arrangement of the spheres is required to produce a play of colour. The size of the spheres determines the colour, with smaller spheres producing the red hue, and larger spheres the purple colour. The colourplay in Mexican opals is often strongly dependent on the intensity of the light in which the stone is viewed, the more valuable stones have colourplay in dim light and the colourplay often takes the form of a rolling flash.
Mexican opal mines exist all over Mexico, many are near the town of Magdalena, where several Mexican opals are sold. One of the largest opal mines is the San Martin mine which produces almost all of the types of Mexican opal described below, many newer mines are appearing all the time and there is much unexplored territory in Mexico which may yet yield opals.
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