Rocks are solids that form Earth’s crust. Rocks contain minerals, which have a definite composition and structure. A rock can be made of one mineral or a(n) mixture of minerals. These minerals are made up of particular elements. It is not made up of living matter. Minerals in rocks can be distinguished by their crystal structure. This is the geometric shape their atoms are arranged in. Different minerals have different properties. One property is hardness, or how easily the mineral can be scratched. The hardest mineral is diamond. Luster, or how a mineral reflects light, is another property. The streak of a mineral is its color when it’s ground into powder.