Carbon, with atomic number 6, is the primary element in diamonds.
A diamond is a crystalline form, or allotrope, of carbon. Each carbon atom is covalently bonded to four neighboring carbon atoms in a rigid three-dimensional lattice. That structure makes diamond exceptionally hard and gives it a high refractive index, which produces its characteristic sparkle when cut and polished.
Carbon also forms very different materials. In graphite, carbon atoms are arranged in flat layers that slide easily, making graphite soft and useful in pencils. Graphite conducts electricity, while diamond generally does not, even though both materials are made entirely of carbon. Other carbon allotropes include fullerenes, graphene, and carbon nanotubes.
Diamonds form deep within Earth, commonly at depths of roughly 150 to 200 kilometers, where high pressure and temperature allow the crystal structure to develop. Volcanic eruptions can carry them toward the surface in rocks called kimberlites. The presence of trace impurities can produce colors such as blue, yellow, or pink. The key distinction is that diamonds are not made primarily of silicon or oxygen: their defining crystal lattice consists of carbon atoms.