Mineral
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure.
Definition
A mineral is a solid material that forms in nature, was never alive, and is made of the same stuff all the way through, with its atoms stacked in a neat, repeating pattern. Geologists formalize this as five tests: a mineral must be naturally occurring, inorganic, solid, of definite chemical composition, and built on an orderly internal crystal structure. That composition can be fixed or can vary within a defined range, as in the olivine series $(Mg,Fe)_2SiO_4$. Minerals are identified by properties such as hardness on the Mohs scale, luster, streak, cleavage, and color. Roughly $6{,}000$ minerals are recognized, but about $90\%$ of the crust by volume consists of silicates built from the $SiO_4^{4-}$ tetrahedron. Bonding type and structural arrangement decide physical properties: graphite and diamond are both pure carbon, yet their different lattices give hardness values of about $1$ and $10$.
Example
Table salt is the mineral halite, and quartz, the clear or milky stones in creek beds around Charlotte, is another. Quartz is $SiO_2$ with a Mohs hardness of $7$, so it scratches glass; calcite is $CaCO_3$, fizzes in weak acid, and breaks along three cleavage directions. North Carolina's Spruce Pine district mines high-purity quartz and feldspar. Feldspars, the most abundant crustal mineral group, form two solid-solution series, alkali feldspar $(K,Na)AlSi_3O_8$ and plagioclase from $NaAlSi_3O_8$ to $CaAl_2Si_2O_8$, and their composition in an igneous rock records the temperature and melt chemistry at the moment of crystallization.
Key Insight
Ice is a mineral when it forms in nature, but water is not, because a mineral has to be solid. Color is the least reliable identification test: quartz alone comes in clear, purple (amethyst), pink, and smoky varieties, but its hardness and crystal habit never change. Mineral stability is ultimately a thermodynamic question. A mineral that crystallized at high pressure deep in the mantle, like diamond, is metastable at the surface and persists only because its conversion to graphite is kinetically slow.