The Devonian was a peculiar time in the Phanerozoic evolution of the Earth. Most continents, including the large Gondwana and Laurussia cratons, formed a Pangea-type assembly around the tropical Prototethys and an increasingly hot, global, greenhouse climate prevailed, with a complete lack of major ice sheets, even in polar areas. There was gradual and increasing flooding of the continents, creating huge epicontinental seas that have no modern analogues. Under these conditions the plants finally conquered the land, with the innovation of deep roots in the Emsian, the appearance of seed precursors and trees in the Givetian, and the spread of vegetation into dry uplands in the late Famennian. In the marine realm, the largestknown Phanerozoic tropical reef belts surrounded craton margins and tropical islands. It was the time of the sudden radiation of early ammonoids, of the earliest episodic blooms of calcareousshelled, pelagic zooplankton (tentaculitoids), the rise to dominance of fishes, mostly of armoured forms and with giants reaching 10 m in length, but also including the first sharks, and the appearance of earliest tetrapods in marginal settings. However, the tropical and subtropical areas reaching up to 458 latitude were hardly a paradise. <...>

The absolute and relative time of subduction of rocks is crucial information for subduction-exhumation models. We investigated the timing of subduction in one of the oldest ultra-high pressure (UHP) localities worldwide: the Kokchetav massif in Kazakhstan. SHRIMP ion microprobe dating of monazite from coesite-bearing micaschists of the Kulet unit indicates that subduction occurred between ~500-520 Ma. This new data provides evidence that the Kulet unit underwent UHP metamorphism 10-15 Ma later than the diamond-facies rocks in the nearby Kumdy-Kol unit. This time constrain excludes models that argue for a simultaneous evolution of coesite- and diamond-facies rocks, it suggest that subduction continued well after continental crust was involved, and that exhumation was not initiated by a single event such as slab break-off. The dynamic of this UHP massif also indicates that Cambrian tectonic was similar to that of recent orogenic belts.

Diamond deposits can be classified as primary (kimberlites and lamproites) and secondary (alluvial and marine). In 1995, their relative production contributions in South Africa were, primary (kimberlite) 8.63 million carats (89% of total), alluvial 960,000 carats (10%). and marine 90.000 carats (1%). The percentage of gems in kimberlites is highly variable. Published figures suggest approximately 40% for the Kimberley mines and 55% for Premier mine. The proportion of gems in the West Coast marine deposits is over 98%. The MIBA mine in the Congo and the Argyle mine in Australia have an average gem content of only 5%. The dispersal of diamonds from their primary sources into streams and rivers and ultimately to the sea is generally accompanied by an increase in average value per carat.| as flawed stones are progressively destroyed with greater and greater transport.

The worldwide acceptance of the first edition of this book encouraged me to extensively revise and extend the second edition. The book was of value to readers of widely ranging interests as demonstrated by the letters received from scientists, students, mining engineers, editors of periodical papers and teachers.

This revision comes five years after the publication of the first edition. Many entries have been improved and now include new data. In total it includes about 25 000 entries, 1 500 graphic figures and 42 tables.

The first edition was criticized by readers who felt that some entries were not related to respective materials: but I would like to emphasize that many minerals are described in the form of references necessary for determining other gemstone materials.

In this edition chapters on light, color and colorant are dealt with in more depth, a large section introduces new terms applying to these areas.

If you have criticisms or suggestions please feel free to contact the author.