According to the chemical build of the stone, Diamonds are separated into four types: Type Ia, Type Ib, Type IIa and Type IIb. Diamonds are made of carbon, and are extremely pure, but in almost all diamonds there are tiny proportions of other elements, interspersed within the carbon as part of their crystal structure. These "impurities" are not what are known as inclusions, and are so small as to be invisible even under a very powerful microscope.
Interestingly, one stone can actually be classified as more than one specific type of diamond, as is the case in most stones in the market today.
Type I Diamonds:
Type I Diamonds are the most common. They represent 98% of all natural diamonds and have detectable traces of Nitrogen.
Type Ia Diamonds:
It contain clusters of Nitrogen atoms throughout the crystal structure of the stone. They tend to emit a yellowish tone.
Type IaA: where the nitrogen atoms are in pairs; these do not affect the diamond's color.
Type IaB: where the nitrogen atoms are in large even-numbered aggregates; these impart a yellow to brown tint.
Type Ib Diamond:
It contain Nitrogen atoms as well. These atoms are all singular as opposed to clustered groups. These stones are 0.1% of all diamonds and emit a strong yellow, orange, brown and even green color tone.
Type II Diamonds:
Type II Diamonds are diamonds that absorbed no, or very few, nitrogen atoms.
Type IIa Diamonds:
Type IIa Diamonds are the most valuable and consequently are very collectable items. They contain either very little or no Nitrogen atoms in the crystal structure of the stones. White stones are exceptionally colorless and fancy colored diamonds are often found with a brown, purple, or pink tone. They represent only 1% - 2% of all diamonds.
Type IIb Diamonds:
It contain elements of Boron within the structure. As a result, they often emit a blue or gray tone. They represent only 0.1% of diamonds. And consequently, they are also very valuable.
In short, Type IIa and Type IIb stones are the most unique and the “purest of pure”.
Green diamonds are a separate case: these diamonds can contain clustered nitrogen atoms or they can contain no nitrogen atoms - what gives them their color is that they have been bombarded by nuclear rays during their growth. This bombardment makes them absorb magenta wavelengths, which gives them their green color. These diamonds are extremely rare.