The 4C's of Coloured Gemstones

Colour, clarity, cut and carat, are also known as the 4Cs. While the concept of the 4Cs was developed as a global language to grade and evaluate diamonds, they also serve as a means to study coloured gemstones

Colour:

For all gemstones except Diamonds, colour is the most important factor in determining quality. There are four of these to consider when buying a coloured gemstone:

Hue (position on a colour wheel)
Saturation (intensity)
Tone (lightness or darkness)
Colour coverage (percentage)

Clarity:

Clarity is judged by reference to inclusions. Magnification can be used to locate inclusions, but with the exception of inclusions which might impact durability, only those visible to the naked eye should influence the final grade. In this way, coloured gems are very different from diamond.

Cut:

Evaluation of cut involves five major factors (in no particular order):

Shape
Cutting style
Proportions
Symmetry
Finish

Carat Weight:

Weight in gems is calculated in metric carats, where five carats equal one gram. Generally, as a gem’s weight increases, so does the per-carat price.