The absorption spectrum of the nitrogen clusters can cause the diamond to absorb blue light, making it appear pale yellow or almost colorless.
Most Ia diamonds are a mixture of IaA and IaB material; these diamonds belong to the Cape series, named after the diamond-rich region formerly known as Cape Province in South Africa, whose deposits are largely Type Ia.
They contain up to 0.05% (500 ppm) of nitrogen, but the impurities are more diffuse: the atoms are dispersed throughout the crystal in isolated sites.
Type II diamonds were formed under extremely high pressure for longer time periods.
These imperfections can confer a yellow, brown, orange, pink, red, or purple colour to the gem.
Many famous large diamonds, like the Cullinan, Koh-i-Noor, Lesedi La Rona, and The Lulo Rose are Type IIa.
The absorption spectrum of boron causes these gems to absorb red, orange, and yellow light, lending Type IIb diamonds a light blue or grey color, though examples with low levels of boron impurities can also be colorless.
Not restricted to type are green diamonds, whose color is derived from exposure to varying quantities of ionizing radiation.