Minerals are the materials that make up the rocks of the
Earth’s crust. Among thousands of different minerals, only a few,
including quartz, feldspar, and calcite, form most rocks. Native minerals, such
as gold and copper, contain one chemical element. Compound minerals, such as
quartz, contain two or more chemical elements. Most minerals are made up of
CRYSTALS and can be described by their properties.
Colour, lustre, and habit
The colour of a mineral’s crystal, its surface shine
(lustre), and the form (habit) made by a group of its crystals.
The colour of a streak of powder left by the mineral when it is
rubbed across an unglazed porcelain tile.
The lines of weakness along which the mineral breaks easily when it
is hit with a hammer.
Resistance to scratching, measured on Mohs scale, going from 1
(talc, very soft) to 10 (diamond, very hard).
The basic geometrical shape that a crystal of the mineral grows
into. There are six crystal systems.
In rock, minerals are normally found as tiny grains. It is harder to
identify minerals in rock than when they are in the form of large crystals
because the grains are often very small and do not have typical crystal shapes.
One method used is to view a thin slice of rock under a microscope in polarized
light. The minerals then show up in different colours, as in this piece of mica
schist.
Minerals come in the form of crystals. This means that
the mineral’s atoms (small particles) are arranged in a regular way, in
neat rows and columns. The arrangement is not obvious when you look at mineral
grains in rocks, but a free-growing crystal always forms a geometric shape,
with flat faces. A crystal is normally symmetrical. All crystals form in one of
six systems of symmetry, which help identify them.
A gemstone, such as garnet, is a crystal found in rock, that is
used for jewellery or other decoration. Raw (uncut) gemstones are cut and
polished before they are used. Gemstones are also called precious stones
because of their beautiful colours, hardness, and rarity. The most prized
gemstones include diamond, ruby, sapphire, and emerald.
