Purple, the most regal of colours, is a combination of red and blue. However Purple diamonds are visually closer to red in the colour spectrum. Lilac, as it may also be called, in diamonds is a hue commonly associated with pink. In lighter tones, it can be challenging to distinguish these two colours. Pink when present adds brightness to purple diamonds shifting them into mauve and lilac colours (reddish purples). Deeper, purer purples include amethyst, grape or plum colours. Nice pure purple to violet is indeed very rare! Warm purples combine pink (really red) and brown, which is not appreciated. Cool purples, on the other hand, contain blue and grey secondary modifiers.
The vast majority of purple diamonds are Type Ia containing nitrogen in aggregate form that has experienced post-growth plastic deformation of the crystal. However, unlike in pink or brown diamonds, purple graining is usually much thicker and more widely spaced and typically only observed in one direction parallel to the octahedral ( natural cleavage) plane in a diamond. This phenomenon causes the typical absorption lines between 550Nm and 590Nm, which helps distinguish such diamonds.
A noteworthy oddity is extremely rare Type IIb stones showing pervasive red phosphorescence turning them Purple.
The most notable producer of distinct purple diamonds currently remains the Alrosa operated Mir mine in Siberia. However also other Mines in Jakutia, Russia, the Argyle mine Western Australia as well as occurrences in Brazil and South Africa are known for their purple beauties.