Each mineral is a distinct, inorganic compound with
a definite chemical composition and atomic structure.
Most of the time, rocks are aggregates composed of various
minerals, though one mineral can be dominant over the
others. Rocks typically fit into three general classes:
igneous rocks, sedimentary rocks and metamorphic rocks.
Igneous rocks are formed by the solidification of a hot
molten substance (magma) that is pushed into the fissures
in the Earth’s crust or erupts from the
surface in the form of volcanic lava. When the magma,
which is composed of rather fluid elements, makes its
way into the fractures in the rocks, it forms the gemmiferous
pockets that contain the best gem-producing materials.
This type of crystallisation is called pegmatitic. The
process of sedimentary rock formation is divided into
three stages. The first stage consists of the meteoric
degradation of the mother rock due to the actions of
atmospheric agents, temperature variations or the chemical
alteration of less stable materials. The second stage
involves the transport of the products of the meteoric
degradation (deposits, dust and fragments) by water or
wind. The third stage consists of the sedimentation of
the deposits. Usually, the selection of the deposits
is carried out naturally. The “lighter” deposits
are carried by currents, while the more gemmologically
important and “heavier” minerals are deposited,
in the form of smooth and polished pebbles, in depressions
in the basins of waterways to form drifts or alluvial
deposits. The consolidation of these sediments forms
stratified rocks. This process of formation is typical
for stones such as turquoise, malachite and opal.
The heat and pressure stemming from intrusions of igneous
rocks or the collision of rocky masses during disturbances
deep in the earth’s crust can modify the igneous
or sedimentary rocks involved in the phenomenon. As a
result, a new type of rock, called metamorphic rock,
is formed. The primary cause of this transformation is
high temperature, though another possible influence is
the gas emitted during the final stages of the cooling
of the igneous intrusion. |
| Almost all minerals grow in regular, symmetrical
crystal arrangements.
- The following seven crystal systems exist in crystallography:
- Isometric or Cubic e.g. diamond, garnet
- Tetragonal e.g. zircon
- Hexagonal e.g. amethyst, emerald
- Trigonal e.g. dioptase, ruby
- Orthorhombic e.g. iolite
- Monoclinic e.g. moonstone
- Triclinic e.g. labradorite
|
