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:

Colour vs. clarity in gemstones involves a trade-off: Colour defines the gem's hue, tone, and saturation (more vivid is usually better), while clarity assesses internal flaws (inclusions) or external blemishes.  Fewer imperfections generally increases value, though some gems (like emeralds) often have many inclusions, and certain types (like Sunstone) can have unique inclusions that add value.

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 diamonds.

Cut:

A well-cut gemstone is crucial because it unlocks a stone's inherent beauty by maximizing it's brilliance and fire making it sparkle, display better colour and therefore significantly boosting its aesthetic appeal and market value. 

Precision cutting optimises light reflection, hides imperfections, influences colour, and transforms raw material into a captivating jewel, whereas a poor cut results in a dull, lifeless stone. 

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.