The occurrence of diamonds in iron–magnesium silicates in the volcanic structures called pipes and in iron–nickel and iron sulfide phases in meteorites suggests that they were formed by dissolution of carbon in those compounds and subsequent crystallization from them in the molten state at temperatures and pressures favourable to diamond stability.
The successful synthesis of diamond is based upon this principle.
Of the stable nuclides, the isotope carbon-13 is of particular interest in that its nuclear spin imparts response in a device called a nuclear magnetic resonance spectrometer, which is useful when investigating the molecular structures of covalently bonded compounds containing carbon.
Thus, the extreme hardness, the high sublimation temperature, the presumed extremely high melting point (extrapolated from known behaviour), and the reduced chemical reactivity and insulating properties are all reasonable consequences of the crystal structure.
Because of both the sense and the direction of the tetrahedral axis, four spatial orientations of carbon atoms exist, leading to two tetrahedral and two octahedral (eight-faced) forms of diamond.
When an element exists in more than one crystalline form, those forms are called allotropes; the two most common allotropes of carbon are diamond and graphite.
The crystal structure of diamond is an infinite three-dimensional array of carbon atoms, each of which forms a structure in which each of the bonds makes equal angles with its neighbours.