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Each gem deposit-whether of primary origin in the parent rocks; or secondary as alluvial placers in valley floors, river gravels, or the sand of oceanic shelves presents an eloquent chronicle of the Earth´s life story. It reveals to the expert the prodigious processes which formed the present crust of our planet, of which this volume discloses a small but exciting detail. The materials of the Earth´s crust are the rocks. In this book, the author expounds on how they were formed, why they altered, why they became the cradles of precious gemstones, how they are categorized, and how they are now exploited by man. What initiates the growth of gemstones? How do they crystallize? Why do gemstones of the same species, originating from different sources, vary? What causes the occurrence of varieties? Why do diamonds, unlike other precious stones, occur not near the Earth´s surface in its crust, but deep down beneath it in the upper mantle? These are only a few of the entrancing subjects discussed in this enlightening volume. The reader learns that the Earth is surprisingly alive and altering constantly-sometimes through slow and equable changes and at times by violent and tremendous cataclysms, events from which gemstones issue.

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I Gemstones Deposited by Water on the Earth´s Surface.- 1. Gemstones Concentrated by Surface Waters: The Gem Gravels of Sri Lanka.- 2. Gemstones Formed from Surface Water: The Opals of Australia.- II Gemstones of Igneous-Hydrothermal Origin.- 3. Hydrothermal Gem Deposits: The Emerald Deposits of Colombia.- 4. Gemstones Formed in Pegmatites: Gem Pegmatites of Minas Gerais, Brazil.- 5. Gemstones Formed Directly from Molten Rock: The Ruby Deposits of Chanthaburi-Trat, Thailand.- III Gemstones Formed by Very High Temperatures and Pressures.- 6. Gemstones Formed by Low-Pressure Regional Metamorphism: The Ruby Deposits of Mogok, Burma.- 7. Gemstones Formed by High-Pressure Regional Metamorphism: The Jadeite Deposits of Tawmaw, Burma.- IV Gemstones Formed at Great Depths.- 8. Mantle Thrust Sheet Gem Deposits: The Zabargad Island, Egypt, Peridot Deposits.- 9. Diamond Pipes: The Diamond Deposits of Argyle, Western Australia.

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