Diamond type classification is based on the presence or absence of nitrogen and boron impurities and their configurations in the diamond lattice. These schematic diagrams illustrate the manner in which N and boron atoms replace C atoms in the diamond lattice. The diagrams are simplified twodimensional representations of the tetrahedrally bonded C atoms (four bonds per C atom) that form the threedimensional diamond crystal structure. Type Ia diamonds contain aggregated N impurities—including A-aggregates (IaA), which consist of pairs of N atoms, and B-aggregates (IaB), which are made up of four N atoms around a vacancy (V). Type Ib diamonds have isolated N atoms. Type IIa stones contain no measurable impurities, and type IIb diamonds have boron impurities.