Introduction

Opal is currently being mined from depths between 1 metre (3 feet) and 30 metres (90 feet) below the surface. The opal is found in sedimentary rocks that were deposited in a freshwater environment during the Cretaceous period, about 110 million years ago. There are more than 200 distinct opal fields on the ridges of Cretaceous rocks surrounding the town of Lightning Ridge. These opal fields may be isolated or occur in groups. The main opal field "groups" are located at Lightning Ridge, Coocoran, Grawin/Carter's, Glengarry/Sheepyard, Wyoming, Jag Hill and Mehi.

A number of theories, or models, exist for how opal was formed in the Lightning Ridge region.

Types of Opal

Precious opal is the valuable form of opal which shows colourful patterns that result from the diffraction of light in a process similar to how the colours of a rainbow are formed. Good quality opals will show some variation in their patterns and colours as they are viewed from different angles, for example when they are turned in the hand.

Potch, or common opal, is generally of no value compared to precious opal as it does show any diffraction of colour. Common opal occurs far more abundantly in the opal mines than precious opals.

Both precious and common opal is found in three major forms, or shapes:

Nobby opal is a type of opal that is found as rounded nodules from the size of a pea to as large as a potato

Seam opal is usually found as a horizontal layer, or seam, in the opal bearing rocks

Fossil pseudomorphs (false forms) are plant, shell, bone or teeth fossils which have been replaced by opal