Gems, Sixth Edition by Michael O'Donoghue

By Michael O'Donoghue

Because it used to be first released in 1962 'Gems' has develop into universally recognized because the so much finished and authoritative treatise on gem fabrics within the English language. This 6th version, has been revised for relevance to modern-day perform and contains an elevated encyclopedic section.'Gems' caters for the pro jeweller and pupil gemmologist, collector of gem stones and people with an insatiable curiosity in worthwhile stones. * Descriptive account of all gem fabrics used as decoration, besides their man made opposite numbers * Contributing authors, on the best of the sphere, convey the newest advancements to the reader* the single encyclopedic reference for gem stones

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The movements of hydrothermal fluids, pore fluids from sediments and groundwaters derived from weathering (meteoric waters) combine to provide a powerful mechanism to concentrate elements and deposit minerals. In the mineral world, gold in quartz veins, copper sulphides in igneous rocks and lead sulphides in sedimentary rocks are examples of economically important hydrothermal deposits. Such orebodies can themselves 27 The Geological Sources of Gems subsequently be re-worked by groundwater, and around copper deposits, turquoise may be formed.

At low grade, the foliation in slates and phyllites may be difficult to see, but at higher grade, the foliation becomes more conspicuous as the micas and other minerals are coarser and easier to see. If such rocks contain more than 50% mica, they are called schists. With less than 50% mica, these medium and high-grade rocks tend to be more massive and less fissile and are called gneiss. Even though they are less fissile, gneisses may contain black amphiboles and creamy white feldspars in contrasting bands and show strong foliation.

The Witwatersrand quartzites and conglomerates are ancient sediments which contain alluvial diamonds. 28). 28 Diamond-bearing conglomerate at Sopa, near Diamantina, Minas Gerais, Brazil. The conglomerate is Precambrian, probably 1500 million years old, so the diamonds must have come from a source even older Amber The third example chosen to illustrate the many ages of gems is the organic gem, amber. Amber is a resin, derived from trees, which has been preserved and fossilized in sedimentary rocks.

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