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Crystalline
A mineral that has solidified
but has been unable to produce well-formed crystals. Quartz and halite are commonly found as crystalline masses.
Crystallisation
The formation of crystals.
Cubic
A crystal system in which crystals have three axes all at right angles to one another and of equal length.
Current bedding
(See: Cross-bedding.)
Cyclothem
A repeating sequence of rocks found in coal strata. A cyclothem is formed when the sea level rises and falls by small amounts. The result of the changing sea levels
is that a coastal swamp can be easily flooded. This could cause the swamp plants to be buried by a layer of sand or clay. Then, if the sea level falls a little, a new swamp would form. Many coal-bearing rocks show repeating cycles of this kind. This is why many coal seams are thin, and why coal mines have to exploit many levels in a mine.
D
Desert soil
A poorly formed soil in a desert region. Because rainfall is irregular, little material is washed from the topsoil to the subsoil. With few plants growing in a desert, there
is little humus in the soil, and
the dryness and heat tend to make the humus shrivel up very quickly. Furthermore, when it does rain, the water only seeps a little way into the soil before the heat from the Sun draws it back to the surface,
 Cyclothem – When there are rapid changes of sea level. Cyclothems always include a coal band.
Peat
bringing dissolved minerals with it. As the water evaporates from the surface, the minerals are left behind and eventually form a salt crust.
Diamond
A form of the element carbon. Diamond is the hardest natural substance known. Pure diamond
is a colourless and transparent mineral. Diamond is not always colourless; if it contains impurities, it may be a darker colour. Some diamonds are almost black.
Diamond is named from the Greek word adamas, meaning invincible. Diamonds are formed under immense temperatures and pressures, such as found below volcanoes. The most famous diamond mine, at Kimberley, South Africa, follows an old volcanic pipe for more than two kilometres vertically into the Earth. Diamonds are also found as placer deposits.
Diamonds commonly form a shape like two pyramids base
to base (a tetrahedron). Jewellers make use of this property when they cut rough diamonds to make jewellery. Each of the faces (called facets) is created by splitting the diamond parallel to the faces of its crystals.
 Diamond – These are diamonds in their volcanic rock setting.
Diamond
Lignite coal
Bituminous coal
Anthracite coal
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