What are Minerals?
A mineral is a naturally occurring, inorganic substance with an orderly internal arrangement of the atoms, and with a chemical composition and physical characteristics that are either fixed or vary within a definite range. By contrast glass has no orderly internal arrangement of its atoms. Therefore, obsidian which is volcanic glass has no minerals.
Let’s compare the minerals diamond and graphite. Both have the chemical composition C (they are both made up only of the element carbon). However, the atoms of carbon are arranged quite differently in each. In graphite they are arranged into hexagons and in diamond they are arranged into cubes. This difference in arrangement of atoms causes drastic differences in physical properties. For instance graphite has the lowest hardness of minerals, and diamonds are the hardest. In other words, all other minerals will scratch graphite; no other mineral will scratch diamond. Graphite is opaque and diamond is transparent. Diamond has a glassy luster, whereas graphite has a metallic luster.
A rock is an aggregate of one or more minerals. For instance, the rock granite is made up of the minerals quartz, mica, and two kinds of feldspar. However, Colorado’s state rock, marble, is made up of only one mineral ― calcite.