Minerals can be identified using a number of properties. These include physical and chemical properties such as hardness, density, cleavage and colour, crystallography, electrical conductivity, magnetism, radioactivity and fluorescence.
This is usually expressed as the main body colour of the mineral, along with the intensity of this colour. Some minerals are of differing colour along different crystal axes, a phenomenon called pleochroism (if the colour varies in two directions, the mineral is called dichroic whereas if the colour varies in three directions the mineral is called trichroic).
Some minerals are always the same colour, such as copper minerals azurite (blue) and malachite (green), but others can show a range of colours depending on different impurity elements or structural defects in their crystal structure (for example fluorite, tourmaline, quartz, corundum). For corundum, chromium causes the red colour in ruby, and iron and titanium cause the blue colour in sapphire.
This is the appearance of a mineral surface in reflected light:
This is the resistance of a mineral surface to scratching. This is a simple scale of relative (not absolute) hardness ranging from 1 (talc) to 10 (diamond). In absolute hardness, corundum (9) is twice as hard as topaz (8) and diamond (10) is four times as hard as corundum. It is a very simple test requiring the scratching of the unknown mineral against a mineral of known hardness. However, minerals can be harder in different directions (e.g. kyanite is softer along its long axis (hardness of 5) than across it (hardness of 7).)
The full scale is:
This relates to the number, direction and intensity of development of regular-spaced breakage directions of a mineral parallel to crystallographic planes. It reflects differing strengths of atomic bonding in different directions in the internal atomic structure. It is usually specified by its quality (perfect, good, fair, poor) and its direction. Members of the mica group have one perfect basal cleavage, feldspars have two well-developed cleavages almost at right angles. Some minerals lack cleavage (e.g. quartz, garnet) and instead have an irregular fracture surface. Pyroxenes have two distinct cleavages at 90°, amphiboles have two distinct cleavages at 120°. Galena (lead sulfide) has cubic cleavage, and fluorite (calcium fluoride) has octahedral cleavage). Parting is similar to cleavage but is breakage along planes of structural weakness such as twinning planes, caused by external stresses.
Density is the mass per unit volume but is inconvenient to measure directly. Instead, we more commonly use specific gravity, a relative density in which the mineral is weighed in air, followed by a weighing while the mineral is immersed in water (density of pure water is 1). It is a ratio between the weight of a mineral and the weight of an equal volume of water.
The specific gravity (SG) is calculated from weight in air divided by loss of weight in water, expressed in units of grams per cubic centimetre. The specific gravity of most minerals ranges from 1.5 up to 19.5.
This is the colour of the powdered form of a mineral. It is a more reliable indication of a mineral than its main body colour, as it is more constant.
It is usually tested by drawing the mineral across an unglazed porcelain tile (streak plate) to leave a coloured streak.
Transparency (diaphaneity) is the degree to which light is transmitted through a mineral. Minerals can be opaque, translucent, or transparent.
The quality of the development of crystal faces present:
