A mineral is naturally occurring, inorganic (it has not lived), crystalline solid, with a definite chemical formula (or formula range). In contrast, a rock is generally a combination of minerals or organic remains, and therefore does not have a unique, specific, or consistent chemical composition.
There are over 5,421 known mineral species. Over 5,208 of these have been approved by the International Mineralogical Association (IMA). It is worthy of note that since silicon and oxygen constitute approximately 75% of the Earth's crust, the 1000 silicate minerals comprise over 90% of the minerals of the crust-- the two most common being quartz and feldspar.
Minerals are identified by their physical and chemical properties:
*crystal structure (there are 7 systems --isometric, tetragonal, orthorhombic, hexagonal, rhombohedral, monoclinic, and triclinic)
*crystal form (cubic, tetrahedral, rhombohedral...)
*habit (blocky, arborescent, acicular...),
*color,
*luster (how a mineral reflects light--adamantine, vitreous, resinous, greasy, metallic...,)
*hardness (a measure of a mineral's scratchability),
*cleavage (a flat, repeatable break determined by the atomic structure) or
*fracture (irregular, non-repeatable break characteristic of minerals with bonding equal in all directions),
*tenacity (a description of a mineral's resistance to breaking or deformation-- brittle, malleable, ductile, sectile, elastic, plastic)
*diaphaneity (how a mineral transmits light-- transparent, translucent, opaque),
*specific gravity (a comparison of a mineral's weight to that of an equal volume of water) , and
*special characteristics: magnetism, reaction to acid, taste, smell, radioactivity
*chemical composition (there are 6 basic chemical classes of minerals - native elements, sulfides, oxides, halides, carbonates, silicates, phosphates)