In Part 1 (Minerals explained 43, Geology Today 2006, v.22, no.2, pp.71–77) graphite was examined, the polymorph of carbon that is stable over a wide temperature range, but only at relatively low pressures. The other principal polymorph of carbon, diamond, is dealt with here in Part 2. Diamond has a very large stability range over both temperatures and pressures, although it is created at similar depths in the Earth's crust, probably in the mantle (Fig. 1). It would probably have remained there unsuspected, had it not been brought to the Earth's surface by volcanic mechanisms. This will be looked at in detail in the section on the genesis of diamond below, as will the apparently anomalous stability of diamond at NTP.