First Field Guide to gemstones of Southern Africa, by Bruce Cairncross
First Field Guide to gemstones of Southern Africa, by Bruce Cairncross.
Introduction by Bruce Cairncross: What are gemstones? Most are types of naturally occurring mineral or rock.
Some, such as pearl and coral, however, occur organically, which means that they are produced by living matter (plants or animals). In order for a substance to be defined as a gem, it must fulfil three main criteria. Firstly, it must be beautiful.
This usually means that it has an attractive colour and relates to the way it reflects light. It also has to be durable and rare. Gemstones are usually cut and polished, and are used for personal adornment or ornamental purposes. In the past the terms precious and semi-precious were used to classify gems. The subdivision led to confusion in terminology and to the view that semi-precious stones were inferior. For these reasons, the subdivision is now discouraged and all substances that are rare, durable and beautiful enough are called gemstones. Precious stones (diamond, ruby, emerald, sapphire, pearl and sometimes opal and alexandrite) are rare and considered beautiful. Emerald Semi-precious stones (aquamarine, topaz, tourmaline and amethyst) are more common and less expensive. Southern Africa contains a wealth of diverse rocks and minerals.
Some of the worlds oldest rocks, which formed over 3 000 million years ago, are found here. Gemstones occur in three types of rock systems: igneous, metamorphic and sedimentary. Most gemstones can only be found in very specific rock types, which is what makes them so hard to find. A knowledge of rock types increases the chances of finding gemstone deposits. Some gemstones, such as diamond, carnelian, agate, chalcedony and jasper, can also occur in alluvial sediments of rivers, such as the Vaal and Gariep (Orange) rivers in South Africa.
This is an extract from the book: First Field Guide to gemstones of Southern Africa, by Bruce Cairncross
Book title: First Field Guide to gemstones of Southern Africa
Authors: Bruce Cairncross
Struik Publishers
Cape Town, South Africa 2001
ISBN 9781868725991
Softcover, 11x17 cm, 56 pages, throughout colour photos
Cairncross, Bruce im Namibiana-Buchangebot
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