By Michael O'Donoghue
Because it used to be first released in 1962 'Gems' has turn into universally acknowledged because the such a lot finished and authoritative treatise on gem fabrics within the English language. This 6th variation, has been revised for relevance to brand new perform and comprises an multiplied encyclopedic section.'Gems' caters for the pro jeweller and pupil gemmologist, collector of gem stones and people with an insatiable curiosity in valuable stones. * Descriptive account of all gem fabrics used as decoration, in addition to their artificial opposite numbers * Contributing authors, on the most sensible of the sphere, carry the newest advancements to the reader* the single encyclopedic reference for gem stones
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Extra resources for Gems, 6th Edition 2006
14 Alluvial sands containing diamonds near Sopa, Minas Gerais, Brazil The above discussion has concentrated on clastic sedimentary rocks – those consisting of fragmentary material; now those formed by evaporation of water and precipitation (chemical sediments), by the actions of organisms (biochemical sediments) or of mixed origin will be outlined. Gypsum, halite and other salt deposits crystallize from evaporating seawater and from lakes in desert regions that have no river outlets. During evaporation, the dissolved salts become more concentrated and eventually the liquid becomes saturated and then supersaturated.
Now it is thought that fluids from the regional sedimentary succession (comprising detrital and chemical deposits – sediments and evaporites) have reacted with organic matter in the black shales to release beryllium, chromium and vanadium. 25 Emerald being prized from a vein in black shale at Chivor, Colombia elements have then joined other elements in the fluids, including silicon and aluminium derived from the shales, to form emerald. Studies of the fluid inclusions in the emeralds indicate that they formed in the veins at temperatures between 200 and 400 °C and at depths of about 4 km.
Like ruby, diamond is another gemstone whose true geological source has been obscured somewhat by volcanic activity, in this instance, by that of kimberlites. But although gem diamonds did not crystallize directly from the kimberlites we see, they are present in eclogite and peridotite xenoliths also carried to the surface by the volcanism. 21) which has formed at depths of more than 35 km (equivalent to more than 10 000 atmospheres pressure) and at temperatures between 400 and 800 °C. The other major source of diamond in these temperature and pressure conditions is peridotite, which is an ultramafic igneous rock consisting mostly of olivine and pyroxene, with only minor quantities of diamond, pyrope garnet or other minerals.