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.
Elizabeth Taylor Diamond
There is a supplemental letter from the Gemological Institute of America stating that the diamond has been determined to be a Type IIa diamond. Type IIa diamonds are the most chemically pure type of diamond, and often have exceptional optical transparency. Type IIa diamonds were first identified as originating from India, particularly from the Golconda region, but have since been recovered in all major diamond-producing regions of the world. Famous examples of Type IIa diamonds are the 530.20 carat Cullinan I and the 105.60 carat Koh-i-noor.
The supplemental letter from the GIA is accompanied by a monograph from the Gemological Institute of America which features additional photographs, data collection charts and gemological research which details the rarity of the Elizabeth Taylor Diamond.