The "Colour" in the 4Cs diamond grading system is an indicator of how colourless a diamond is. According to GIA, "diamonds are colour-graded by comparing them to stones of known colour under controlled lighting and precise viewing conditions". 

Sapphires come in every colour you can imagine. Every single sapphire has its own unique colour. And describing this colour is not a simple task. The colour characteristics of sapphire can be further broken down into three colour properties; hue, saturation, and tone.

Hue is simply the obvious colour difference you see in the stone. There are numerous naming conventions to identify different hues such as navy blue, deep blue, sky blue, denim blue etc. However, there is no consistency in these naming conventions. 

Saturation is how well the colour is distributed across the body of the gemstone and how vibrant the colour is expressed. The best stones have a vivid saturation and the opposite is a dull saturation.

Tone is the relative lightness or darkness of the colour. For example, two sapphires can have the same colour but one can be lighter and another can be darker based on how much grey colour is mixed with its main hue.

As with all precious gemstones the overall mass of each stone is measured by it's carat weight.

Gemstones of below one carat weight may also be expressed in terms of "points." For example, a half carat stone is the same as a "fifty pointer."

Diamond clarity is the absence of any inclusions and blemishes. Natural gemstones, especially sapphires have inclusions. In contrast to diamonds, some inclusions can add value to the sapphires by enhancing the beauty of the gemstone. Therefore, clarity is another complicated characteristic of sapphires.

If inclusions are not visible to the naked eye we call them eye-clean sapphires and if any inclusions are not visible with a 10X magnification we call them loupe-clean sapphires.

Typically the clarity of a sapphire is categorised as being a Type 1 Stone (eyeclean), Type 2 (may contain some inclusions visible to the naked eye, but not enough to distract from the beauty of the overall appearance) and Type 3 (contain inclusions usually noticeable to the naked eye).

The quality of the cut of sapphire is also not as same as the cut or a round brilliant diamond where the GIA grading is defined. The ultimate idea of grading the cut of a gemstone is to determine the glitter and brilliance that comes off the gemstone when interacts with light rays. It is very difficult to establish a clear guideline as to how the geometric dimensions and proportions of the cut lead to the visual appearance of sapphire because, unlike diamonds, this characteristic highly interacts with the colour and clarity of the sapphire.

Sometimes sapphire gemstones are cut out of proportions resulting in the gemstone becomes deep or shallow to maintain and get the optimum color. In such circumstances, the perfection of geometry and symmetry does not mean a better cut.

Curing refers to various treatments and enhancements carried out to improve the properties of sapphires as well as hiding or healing the defects. The most common sapphire treatment is heat treatment.

Heat treatments are the process of exposing the gemstones to high heat for a long duration so that it speeds up the transformations which will take millions of years to happen naturally. Heating is an accepted treatment for sapphires as, except for very few, the majority of sapphires are heat treated.

Undesirable techniques such as filling and dying are used by some but these sapphires are not valued in the trade.

The last "C" of the Sapphire 6C's is the country of origin. Sapphires from different regions have different characteristics. Not only the characteristics but also other factors such as whether the mining is carried out in an environment-friendly manner or whether the trade is conflict-free can be important in the trade.