What are minerals?  Minerals are naturally occurring, inorganic
solids with a definite chemical composition and a regular atomic
structure. From this basis come all their physical properties.
Some of the most obvious physical properties of a mineral are
hardness (resistance to scratching), tenacity (resistance to
impact), specific gravity (relative density), cleavage (the
tendency of some crystals to split along planes of weakest
atomic bonding), magnetism, conductivity of electricity,
reactibility to acids etc. These measurable physical properties,
along with a number of optical ones such as lustre, refractive
index and selective absorption of light rays, enable minerals to
be positively identified by scientific instruments.  The table
below is a universally accepted standard of the comparative
hardness of minerals. It is called Mohs' scale, after Frederich
Mohs, the mineralogist who devised it. The range is from the
softest mineral (talc) to the hardest (diamond).    1. Talc   2.
Gypsum   3. Calcite   4. F