Every Diamond Report issued by the Gübelin Gem Lab is accompanied by a Diamond Type Note. Based on an infrared spectrum, this Note indicates of which type the tested diamond is classified.

Diamonds are classified into two fundamental groups based on the relative presence or absence of nitrogen incorporated into the crystal structure, as determined by the infrared spectrum. Type I diamonds contain appreciable concentrations of nitrogen, whereas type II diamonds are chemically very pure and do not reveal infrared absorption characteristics related to nitrogen.

A further separation of these two groups includes type Ia (nitrogen atoms present in pairs or groups), type Ib (isolated nitrogen atoms), type IIa (no measurable traces of nitrogen) and type IIb (traces of boron).