Gemstones quality is graded using the 4Cs.
Colour
Color is the most important contributing factor to the value of coloured gemstones. Generally strong saturation (intense and vivid colours), and medium tone, are more favored as compared to pastel colours. This ‘C’ is measured by the hue, tone and saturation. Let’s explore what they mean.
Hue
The basic colour of the gemstone is referred to as the hue. It can be described as a single hue or combination of hue. For example a blue sapphire with a tint of violet will be referred to as ‘Violetish Blue’
Tone
How light or dark a stone’s color is referred to as the tone of the gemstone. Medium to medium-dark tones are the most preferred tones for gemstones.
Saturation
This is a key component in determining a sapphire’s value. Regardless of the sapphire’s hue, higher levels of saturation are preferred. Vivid saturation is the most preferred.
Clarity
Clarity refers to the absence of inclusions in the gemstone. The cleaner the gemstone the higher the value. Inclusions can also hinder the brilliance of the gemstone and in this case the value will be affected.
Some examples of inclusions in gemstones are:
a) crystals
b) feathers
c) silk inclusion
d) fractures
When you are not able to detect any inclusions with the naked eye, we call that ‘Eye Clean’. An eye clean stone is considered optimal and if it has no inclusions viewed via a 10x loupe or magnification (industry standard) it is considered as a ‘loupe clean’ piece. Very similar to the diamond grading for clarity we use the following scale for coloured gemstone:
IF – Internally Flawless
VVS – Very Very Slight Included
VS – Very Slight Included
SI – Slightly Included
I – Included
Cut
This is the most important factor for diamonds, where brilliance is what buyers are looking for. Usually based on Marcel Tolkowsky of an ideal cut.
Gem cutters have the following guiding principles when they facet sapphires:
Have sufficient depth for the gemstone to ensure the brilliance of the gemstone is not compromised.
Cut the gemstone to retain as much carat weight as possible.
If the gemstone is cut too shallow, a window will appear and this will cause the light to pass through the gemstone instead of bouncing back out. If the gemstone is cut too deep, extinctions occur where those regions look dark. The right depth will allow the perfect brilliance of the gemstone exhibiting the liveliness of the colour.
Chatoyancy is the technical term for the thin bar of reflected light on certain gemstones, which we commonly refer to as cat’s eye. Fittingly, the word chatoyancy derives from the French phrase for “eye of cat”, or “oeil de chat”. The cat’s eye effect occurs when fibrous or needle-like inclusions form in a parallel direction.
Asterism is the technical term for when light is reflected in a star shape on a gemstone. This star may have four, six, or more rarely, twelve rays. Generally, the more rays the star has, the more valuable the stone. Asterism occurs when rutile (a kind of mineral) needles are included in the gemstone in multiple parallel directions. This causes the reflected beams of light to crisscross in a star shape.
Carat
Carat refers to the weight of the gemstone. As with any gemstone, the per carat price increases with the overall carat weight. This means that the per carat price for the 1ct range will be cheaper than the per carat price for the 2ct range.
This is because it becomes increasingly difficult to find very good quality gemstones in bigger carat size. You can expect exponential increase of the price with the 3ct range, 5 carat range and 10ct and over range.
