Diamonds are composed of carbon atoms arranged in a rigid crystal lattice. This type of lattice arrangement gives diamonds their extreme hardness and thermal conductivity, as well as their high refractive index. In terms of state of matter, diamonds are generally classified as solids due to their strong interatomic bonding and fixed shape. However, at extremely high temperatures, diamonds can transition into a plasma state, where the electrons are completely separated from the carbon atoms. This occurs at temperatures above approximately 3,700 degrees Celsius.