Gems : their sources, descriptions and identification by Michael O'Donoghue

By Michael O'Donoghue

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Such combinations include: the action of granitic or hydrothermal fluids on dolomite, the action of serpentine fluids on silica-rich rocks, or the action of hydrous fluids on serpentinite. 25 The Geological Sources of Gems In the Merelani region of Tanzania, veins containing tanzanite and tsavorite garnet have resulted from pegmatitic and hydrothermal fluids reacting with medium- to high-grade metamorphic rocks along the crests of folds. 24). The zone of mineralization is only a few metres wide but extends for about 9 km and includes other spectacular gems such as chrome tourmaline and chrome diopside.

The Monte Somma crater wall is in the background Appearance and texture are the most obvious features to use in classifying igneous rocks, but rocks of similar texture, whether coarse or fine, may have very different compositions, different origins and different histories. 2 mm across in a groundmass of pale brown glass and tiny grains of magnetite. Island of Mull, Scotland igneous rocks was based on their silica (SiO2) content; most igneous rocks contain between 40 and 70% silica, and this range gives considerable scope for discrimination.

Sediments and Sedimentary Rocks Rocks of the second major category owe their formation to the actions of air, water, organisms, and biological processes on rocks already in existence. 13). As the layers of sediment accumulate, the older material beneath is compacted and buried. 13 Schematic diagram indicating the broad relationships between Earth’s processes, topography and sediments spaces that remain. At first, sediments are very loosely packed and may be saturated with groundwater but as compaction and pressure increase, the water is squeezed away and the detrital minerals may break down in the presence of water to form new minerals filling the pore spaces and cementing the rock.

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