Beginning in the 1930s, scientists began to recognize that certain kinds of diamonds displayed similar features. They grouped the diamonds into two main categories called type I and type II based on differences in transparency under ultraviolet radiation. Scientists were able to further divide type I and type II diamonds into two subcategories by the arrangement of carbon—and impurity—atoms in the diamond structure. In 1959 they discovered that nitrogen was the principal chemical impurity in diamond and that while type I diamonds contained this impurity, type II diamonds did not.
The vast majority of natural diamonds are what scientists call type Ia. Type Ia diamonds contain plentiful nitrogen in clusters or pairs. This kind of diamond cannot be grown artificially. Type Ib diamonds contain scattered and isolated nitrogen atoms that are not in pairs or clusters. Type Ib diamonds are rare in nature. Type IIa diamond contain almost no nitrogen, while IIb diamond contains boron.
Synthetic diamonds correspond to types Ib, IIa, and IIb, all rare categories among natural diamonds. At GIA type I and type II diamonds can be distinguished by the latter’s transparency under short-wave ultraviolet radiation, and both types can be definitively separated by infrared spectroscopy (The “Type” Classification System of Diamonds and Its Importance in Gemology, Gems & Gemology, Summer 2009, Vol. 45, No. 2).