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By Michael O'Donoghue

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This range of rock types and their inter-relationships are the records of geological processes, and by studying their compositions and properties, their histories can be interpreted. Rocks and the Processes That Formed Them Rocks are composed of minerals, and although over 4000 minerals are known, only about 100 are common enough to be considered as rockforming. 2). Most rocks, however, are composed of two or more minerals. 2 5 Fantiscritti quarry, Carrara, Italy where marble is mined The Geological Sources of Gems A mineral can be defined as a naturally occurring solid, generally inorganic, with a specific chemical composition and crystal structure.

The fluids generated at these depths are mixtures of gases, liquids and solids, and on their turbulent ascent they may incorporate a wide range of rock and mineral fragments from the fractured wall rocks. Consequently the rocks at the surface are mixtures of magma and variable quantities of xenoliths and xenocrysts – the most important of which are diamond. Such rocks were first found in Kimberley, South Africa, and because they did not resemble any currently known rock, they were given the name kimberlite.

The seashells commonly contain two forms of calcium carbonate – calcite and aragonite – and since aragonite is generally responsible for the attractive aspects of shells such as the iridescence of mother of pearl, one might expect limestones to contain areas of similar interest, but during burial, aragonite in the shells progressively recrystallizes to the more stable calcite. During this process the calcite will tend to grow in the pore spaces, cementing and strengthening the rock. Chert, which is popularly known as flint, is also of mixed chemical, biochemical and clastic origin.

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