Only a very tiny percentage of diamond rough production is suitable for cutting and use in jewellery.
In judging the quality–and therefore the value–of a cut diamond, buyers must take into account four criteria, known as the:

Four C's
Colour – Clarity – Carat weight – Cut

Colour
Colourless stones are extremely valuable, while yellow or brown-tinged stones are regarded as imperfect and therefore less valuable.
Many people are also unaware that diamonds can come in every colour under the rainbow and in many different intensities of colour. These are highly prized by collectors and jewellery designers the world over for their phenomenal rarity and beauty.
Coloured diamonds are available on request.

 

Fluoresence in Diamonds

Some people like the effect fluorescence or “photoluminescence” produces in diamonds while others do not. Under normal conditions it is never seen unless the diamond displays a very high fluorescence. In this case, when a stone that displays unusually high fluorescence is exposed to direct or intense ultraviolet light a bluish glow can be observed. Fluorescence is caused by trace elements of boron found in the diamond which when activated by UV light, give you a sometimes blue white colour.
These “blue white stones” were highly sought after in the past as they can seem much brighter than their non fluorescent counterparts. Extreme fluorescence in a diamond can lower the price, but in some cases it can even lift the colour one grade to the naked eye and in recent years GIA have stated that typically fluorescence can be a plus in certain diamonds.
We however rarely buy or sell strong fluorescent diamonds unless we feel them to be of exceptional beauty. Listed below are the abbreviations for describing strength of fluorescence:

• N, NO, NON - No Fluorescence
• F, FB, FT, FA - Faint or Faint Blue Fluorescence
• S, ST, STB, STG - Strong Fluorescence