Diamond type
Diamond type is a method of scientifically classifying diamonds by the level and type of their chemical impurities. Diamonds are separated into four types: Type Ia, Type Ib, Type IIa, and Type IIb. The impurities measured are at the atomic level within the crystal lattice of carbon atoms and so, unlike inclusions, require an infrared spectrometer to detect.
Napoleon Diamond Necklace
While the gems of the Napoleon Diamond Necklace have never been professionally graded by a lapidary (as they have never been removed from their mountings), infrared spectroscopic analysis of the diamonds has shown that they are primarily Type Ia. However, 13 of the 52 largest diamonds in the necklace are of the rare Type IIa variety. A number of the Type Ia diamonds show indications of sulfide crystal imperfections.