but
corundum may occur in great amount in corundum syenites. Although large
bodies of igneous rocks are exposed on the earth's crust in several
parts of the world, most of them do not contain any precious stones.
But in the final consolidation of these large bodies of igneous rocks,
the residual fraction or mother liquor, consisting of liquids and
volatiles, are kept under great pressure and moderate temperatures.
When the pressure could overcome the country rocks, these liquids are
injected through fissures and cracks, which on cooling form coarsely
crystalline rocks known as pegmatite, which forms sheets, lenses,
veins and dykes. Many gem minerals occur in pegmatite dykes with other
less valuable minerals, since materials for forming a number of
minerals and gems are contained in this mother liquor. Steam cavities
also occur in pegmatite dykes, where crystals could grow to great size,
e.g. beryl, topaz, quartz, tourmaline, amazon-stone, etc. Such crystals
can be broken away from the wall of the cavity. The embedded crystals
may also attain enormous dimensions—feldspar crystals weighing 200 tons
were found in Norway as also a spodumene crystal weighing 90 tons in
South Dakota. These pegmatite veins finally grade into vein quartz,
which might also carry some gold and gem minerals.
These
igneous bodies are originally formed at depth but after some ages
become exposed at the surface, when the gem-stones become accessible to
man. Nature lends a great helping hand in this by the processes of
denudation, which wear away large thicknesses of rocks through the
agency of rain, frost, ice, changes of temperature, wind, waves, etc.
Gem minerals may occur in the material thus broken down, either exposed
or carried away.
(b) Sedimentary rocks. The
material worn out by denudation is transported and deposited in water
forming the sedimentary rocks, e.g. clay, shales, sandstone,
limestone, chalk, etc. A few of them might contain some
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