There are a few rocks composed of one mineral, pure quartz, for example, but many rocks are made up of more than one mineral. All crystals are composed of atoms locked together in a certain pattern. The smooth faces of crystals meet in definite angles. Many minerals can have the same chemistry but different crystals because the atoms are linked differently. For example, diamond and graphite are both made of pure carbon, but have different crystal structures.
Diamonds are hard while graphite is soft.
The systems differ from one another in the length and arrangement of the imaginary axes that runs through them. Light is reflected from the surfaces of minerals in various ways, which produces many different types of lustre. Minerals with a metallic lustre are opaque and have a streak darker in colour than the mineral itself. On the other hand, non-metallic minerals become transparent on a thin edge and produce a streak lighter than that of its original colour. One of the most important properties of a mineral is its hardness, which is a measure of its resistance to abrasion or mechanical damage of any kind.