On the periodic table, the element that is the main constituent of natural diamond is carbon.
Diamond is a crystalline form, or allotrope, of carbon. In diamond, each carbon atom is strongly bonded to four neighboring carbon atoms in a three-dimensional structure. This arrangement gives diamond its exceptional hardness and many of its optical properties.
Carbon has atomic number 6 and the symbol C. It also forms other important allotropes, including graphite, in which carbon atoms are arranged in layers. Graphite is soft and electrically conductive, showing how different atomic structures can give the same element very different properties.
Natural diamonds form deep within Earth under high pressure and temperature, then reach the surface through certain volcanic rocks. The answer is carbon rather than graphite: graphite is another form of carbon, not a separate element. Silicon and boron can occur in minerals but do not constitute ordinary natural diamond.