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gemstone
quartz sand
coloured quartz
flint nodules
arrow heads
THE SILICA FAMILY

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 common form of quartz is known by the general term of chalcedony. This is a microcrystalline form; that is it is comprised of a multitude of minute quartz crystallites visible only under a microscope. chalcedony is a hard and tough material which has a variety of uses. It occurs in large quantities in chalk beds as flint nodules which, when broken, are seen to be composed of grey to black material with a thin white coating on the outside. They are particularly well known in the south and east of England where they have been used from time immemorial.

Flint has an excellent conchoidal fracture, so that very sharp edges can be produced by the skilled craftsman. The early Celtic inhabitants of England have left large quarries in East Anglia, where they produced and shaped the flint by skilled chipping and flaking to form stone tools such as axes, scrapers, arrow heads and so on. Being very strong and resistant to weathering, flint has been widely used as a building material where it was abundant. Beautiful flint churches of earlier centuries are still a common sight in the south and east of England.

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.

Silica also forms compounds with other elements in the earth's crust and mantle. These are the silicate minerals, more than 800 of which are known. Most of these materials are uncommon or rare; those making up a major part of rocks in the crust number little more than a dozen. Some of the commonest are feldspar, mica, and olivine. Together with quartz they make up the greater part of our planet outside of the iron-nickel core. The element silicon, at about 28%, is the second most abundant material by volume (after oxygen) in the crust of the earth - that part 20-40 kilometres down from the surface on which we live.

Opal differs from most other forms of silica in that it contains water, the content of which varies from about three to 20%, depending in part on the type of opaline silica. It may be noted that there are other hydrous silica minerals such as silhydrite, but these are rare. What then are these various types of opal?

Our initial question (what is opal?) yielded the answer precious or gem opal, which indeed is the best known opaline material. Although a scientific classification has been proposed for these materials (Jones and the author, R0242), and is given here, we may confine ourselves at this stage to describing the more commonly recognised varieties.