Laboratory
Properties
The Hardness of a mineral is measured on a scale from 1 to 10 depending how easily it scratches. This system was invented by Friedrich Mohs in 1812. Minerals with higher Mohs’ (see below) numbers will always scratch those lower in the scale. So, topaz (8) will scratch calcite (3) but not corundum (9). Minerals can also be scratched with everyday objects: a mineral scratched with a coin will have a hardness of less than 3.5.
Colour is an obvious clue, though some minerals – quartz, for example – come in a large variety of colours. Red ruby and blue sapphire are both types of the mineral corundum. Gold and sulphur are both spectacular yellow minerals.
Lustre (rhymes with General Custer) describes the way daylight reflects off the surface of a mineral. For example, a mineral can be metallic (shining like metal) or dull.
Transparency describes the way in which light passes through a mineral specimen. A transparent mineral allows light to pass through it, like clear glass.
If you crush a mineral into a powder, the colour of the powder is known as Streak (rhymes with beak). This can be quite different from the colour of the uncrushed mineral.
A mineral’s Gravity is calculated by comparing the weight of the mineral with the weight of an equal volume of water. Gravity is shown by an SG number. For example, a mineral with an SG number of 3 is three times heavier than water.