Type II diamonds are divided into types IIa and
IIb (again, see figure 2). Type IIa diamonds contain
no easily measurable N or boron impurities. Natural
type IIb diamonds likewise contain no IR-measurable
N impurities. Instead, type IIb diamonds contain
boron impurities that are thought to be isolated
single atoms that replace carbon in the diamond lattice.
Characteristic properties of type IIb diamonds,
such as electrical conductivity, are a direct result of
the boron impurities.