The primary characteristics of a mineral that determine its physical properties are its composition and the strength of the bonds in its ordered internal structure. Here are some examples:

Galena, a lead sulfide, has a much higher specific gravity than bauxite, an aluminum hydroxide. This difference is because of their composition. Lead is much heavier than aluminum.

Diamond and graphite both consist of pure carbon. Diamond is the hardest natural mineral, and graphite is one of the softest. This difference occurs because of the types of bonds connecting the carbon atoms in their mineral structures. Each carbon atom in diamond is bonded to four other carbon atoms with strong covalent bonds. Graphite has a sheet structure in which atoms within the sheets are bonded to one another with strong covalent bonds, but the bonds between the sheets are weak electrical bonds. When graphite is scratched the weak bonds fail easily, making it a soft mineral.

The gemstones ruby and sapphire are color variations of the mineral corundum. These color differences are caused by composition. When corundum contains trace amounts of chromium, it exhibits the red color of a ruby. However, when it contains trace amounts of iron or titanium, it exhibits the blue color of sapphire. If, at the time of crystallization, enough titanium is present to form tiny crystals of the mineral rutile, a star sapphire may form. This occurs when tiny crystals of rutile align systematically within the crystalline structure of the corundum to give it a silky luster that might produce a “star” that aligns with the primary crystallographic axis.