Coloured Gemstone Grading Factors
Colour
Colour is the most important factor in coloured gems. The more intense the colour the more valuable the stone (not darker, more intense). Colour is described using 3 terms: Hue, Tone and Saturation. Hue is the domiant colour and any additional colours visible in a stone. Examples are ‘orangey-red’ or ‘blueish green’. Tone is the lightness or darkness of the colour, from colourless to black. Saturation is the strength of the colour from grayish or brownish to vivid.
The pure spectral sensations of violet, blue, green, yellow, orange, red and various combinations of these. Hue also refers to the sensation of purple (between red and violet), which is not found in the sun’s spectrum. There are only 31 hues are used in coloured stone grading.
The lightness to darkness of a colour sensation. Tone is divided into seven steps from 2-8.
The strength, purity or intensity of the hue present in a colour sensation. Saturation is assessed on a six level scale.
Clarity
Clarity is the term used to describe how clean the stone is inside using a 10X magnification. The most important thing is how the stone looks to the naked eye in normal (outside and inside) lighting situations.
Clarity for coloured stones will be graded by the same nomenclature as diamonds (FL, VVS, VS, etc). However, some coloured stones naturally grow with more inclusions, therefore, the system is amended depending on the type of gemstone being graded. A TYPE I gemstone grows flawless or nearly so that would be graded strictly like a diamond. A TYPE II gemstone grows somewhat included and inclusions are more common. Grading is more lenient. A TYPE III gemstone is almost always included and grading is most lenient.
Cut
Cut for coloured stones will examine the proportions and finish looking at factors such as outline balance, depth, bulge, windowing, extinction, polish, and symmetry.
A stone that is properly cut is a thing of beauty and brilliance. Look into the gem from the table of the gem (the top) and a well cut stone will be bright across the entire surface. The brightness is the result of the light that enters the stone being reflected back at your eyes. You cannot see through a well cut stone. A poorly cut stone will have what is called a window at the bottom where the light passes through it along with colour and brilliance.
Carat Weight
Carat is the unit of measurement used to describe the weight of the stone. In many gemstone varieties the larger stones are valued significantly higher due to the rarity of these stones compared to the smaller stones of the same quality.