Opal is the birthstone for October. It is a hardened silica gel, usually containing 5-10% water. It is therefore non-crystalline which is unlike most other gemstones. This means that it may eventually dry out and crack. There are two varieties; Precious Opal which shows iridescence depending on the angle it is viewed and there is Common Opal (or Potch Opal) which is usually opaque and displays no iridescence.
The iridescence we see in Precious Opal is due to the regular alignment of tiny silica spheres diffracting light, the larger the spheres the greater the range of colours we see!
Although opal is found throughout the world, almost all of that opal is common opal of very little value. Most of the precious opal deposits that have been discovered are in Australia. The mines of Australia produce at least 90% of the world's precious opal.
Famous mining areas include Australia, United States, Mexico, Hungary, Indonesia, Brazil, Peru, Honduras, Guatemala, Nicaragua, Slovakia, Czech Republic and Ethiopia.