Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At room temperature and pressure, another solid form of carbon known as graphite is the chemically stable form, but diamond almost never converts to it. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are utilized in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.
Because the arrangement of atoms in diamond is extremely rigid, few types of impurity can contaminate it (two exceptions being boron and nitrogen). Small numbers of defects or impurities (about one per million of lattice atoms) color diamond blue (boron), yellow (nitrogen), brown (defects), green (radiation exposure), purple, pink, orange or red. Diamond also has relatively high optical dispersion (ability to disperse light of different colors).
Most natural diamonds have ages between 1 billion and 3.5 billion years. Most were formed at depths between 150 and 250 kilometers (93 and 155 mi) in the Earth's mantle, although a few have come from as deep as 800 kilometers (500 mi). Under high pressure and temperature, carbon-containing fluids dissolved minerals and replaced them with diamonds. Much more recently (tens to hundreds of million years ago), they were carried to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites.
Diamond is a solid form of pure carbon with its atoms arranged in a crystal. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond. The two most common allotropes of pure carbon are diamond and graphite. In graphite the bonds are sp2 orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart. In diamond they are sp3 and the atoms form tetrahedra with each bound to four nearest neighbors. Tetrahedra are rigid, the bonds are strong, and of all known substances diamond has the greatest number of atoms per unit volume, which is why it is both the hardest and the least compressible. It also has a high density, ranging from 3150 to 3530 kilograms per cubic metre (over three times the density of water) in natural diamonds and 3520 kg/m| in pure diamond. In graphite, the bonds between nearest neighbors are even stronger but the bonds between planes are weak, so the planes can easily slip past each other. Thus, graphite is much softer than diamond. However, the stronger bonds make graphite less flammable.
Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness and thermal conductivity, as well as wide bandgap and high optical dispersion. Diamond's ignition point is 720-800 -C in oxygen and 850-1000 C in air.