I. Class of Minerals

Minerals that occur as elements are called native elements, and they are very rare in nature. Around twenty minerals belong to the class of native elements, making up approximately 0.1% of the Earth's crust. The reason for their rare occurrence in nature is that the vast majority of chemical elements easily form compounds.
The class of native elements includes metals, metalloids, and nonmetals. Native metals are composed of dense arrangements of atoms bonded together by metallic bonds. They most often crystallize in cubic or hexagonal systems and have many shared physical properties, such as metallic luster, high density, and good thermal and electrical conductivity. Unlike metals, native metalloids are brittle and cannot be hammered, but they have a metallic luster. They crystallize in a hexagonal system and, due to their layered structure, exhibit good cleavability. Minerals from the group of native nonmetals are few in number, vary significantly, and the best-known among them are diamond, graphite, and sulfur. 

Diamond and graphite are allotropes of the same chemical element-carbon (C), meaning their chemistry is the same. However, they differ in their internal structure, i.e., the arrangement of carbon atoms. In the structure of diamond, each carbon atom is bonded by strong covalent bonds to four adjacent atoms arranged at the vertices of a tetrahedron, while in the structure of graphite, each carbon atom is bonded to three adjacent atoms arranged in the same plane, forming layers that are weakly connected. Consequently, these two minerals differ significantly in their physical properties (shape, color, luster, cleavability, and especially hardness).