Along with cut, clarity and carat weight, colour is one of the four Cs, used to determine a diamond’s value. The facets in a diamond act as a prism, reflecting and refracting the light to create flashes of spectral colour – known as fire. If colour from chemical impurities is present in the diamond, it acts as a filter that lessens the impact of the fire. A perfectly pure diamond should be completely transparent with no trace of colour.
White diamonds with a slight hue are less rare than colourless diamonds. However, diamonds with intense colours such as yellows, pinks or blues are often known as “fancy”, and are very rare and valuable.
The GIA grades colour found in white diamonds from D to Z. Diamonds graded D-F are considered "colourless"; these are the rarest and therefore the most expensive. Stones graded G-J are "near-colourless" and have only a slight colouration that’s invisible to the naked eye. K-M diamonds are "slightly coloured" and those graded N-Y have a distinctly noticeable yellow or brown tinge.
You may come across this term when a diamond’s quality is described, although it is not always mentioned in a grading. Fluorescence refers to the way a stone reacts under UV light. Florescence is graded from None to Very Strong. High levels of florescence can give the stone a slightly milky or oily appearance but the affect is usually very minor. There are instances when high florescence is considered desirable.
Fancy coloured diamonds have their own grading system. They are graded firstly on their basic hue e.g. pink, yellow, blue, green, etc. and secondly on the intensity of the colour. The more vivid and intense the colour the more expensive the diamond is likely to be.
The grading system used for coloured diamonds is similar to that used for other precious gemstones, such as rubies or sapphires, with each stone described using the following terms:
- Fancy Light
- Fancy Intense
- Fancy Vivid