Introduction

Most commonly, opal is known as a gemstone showing many colours in a variety of patterns. As a gemstone, opal has been known for more than two thousand years, but has only become widely available in the hundred years since the discovery of deposits in Australia. However there are many other varieties of opal, all of mineralogical interest, some of commercial and industrial value. The latter types are usually termed 'opaline silica'.

The relationships between these materials can best be appreciated if we understand the nature of opal or opaline silica itself. The last term gives us the clue to its nature. Opal is composed of silica (silicon dioxide), although not all silica is opaline. Silica occurs in many different forms in nature, the best known being quartz, which itself occurs in different forms, ranging from rock crystal and gemstones such as amethyst and citrine, to massive rock formations such as quartzite and sandstone, to sand on beaches and in deserts.

Colourless quartz crystals are well known, have their own intrinsic beauty and may be very large. Coloured quartz is used as a gemstone. Some people believe that quartz crystals have certain healing and magical properties.

Another form of chalcedony which resembles flint to some extent is chert. This often occurs as layers in bedded limestones. Chert has long been used in the ceramic industry. The material is crushed to a fine powder and added to the clay and feldspar when making stoneware and porcelain articles.

Patterned or brightly coloured forms of chalcedony are Less common, and have been used as gem and ornamental materials for thousands of years. Familiar stones in this category are chrysoprase, carnelian, agate and onyx.

Silica also occurs in other less common forms in nature. Many of these are only formed at high temperatures and/or pressures. The better known forms in this category are cristobalite and tridymite, which while not common in natural materials, are very important industrially; they are particularly important in the ceramics industry, especially in relation to refractories.

Occasionally a natural form called melanophlogite forms tiny cubes in volcanic areas. Other rare forms, such as coesite and stishovite, were created by enormous pressures generated in rocks such as those around Meteor Crater in Arizona by the impact of the meteorite. Another form called keatite has been made in the laboratory. In all, more than 20 forms of silica are known.