However, even rocks with the same minerals may look different due to variations in the relative amounts of minerals and the processes by which they are formed. Some rocks may contain minerals in the form of crystals. Crystals have a regular geometric shape. There are 6 basic crystal shapes. A hexagon is the most common shape with six sides, called faces.
Crystals can be small or large.
In other rocks, such as obsidian, the crystal formation is microscopic. The size of the crystals in a rock depend on how fast the rock cooled. Minerals are identified by their hardness. A scratch test developed by a German mineralologist named Fredriech Mohs in 1822 is used to determine hardness. The result of the test is compared to a hardness scale.