The Four Cs: Valuing Diamonds and Other Precious Gems


Diamonds and other precious gems are valued using the four Cs: cut, colour, clarity and carat weight. These factors dictate a stone’s rarity, quality and – ultimately – its price tag.

Every gemstone is unique but gemologists look at four key qualities when carrying out a valuation. While the four Cs are accepted as the universal standard across the world, some factors are more important than others, depending on the variety of stone.

Whether you’re looking to treat yourself or want to ensure you’re making the right decision on an engagement ring, it helps to understand a bit about the four Cs.

Carat weight: is big always better?

Precious gems are measured by weight, using a unit called carats. This is not the same as size. A gem with a shallow cut could look bigger, but weigh less than a gem that has smaller dimensions with a deeper cut.

One carat is about a fifth of a gram. A carat is then broken into 100 points, so you could see a diamond of 1.75 carats or a 0.05 carat emerald.

For a ring with multiple gemstones, the total carat weight will be given along with the carat weight of the individual stones.

Unsurprisingly, carat weight is an important indicator of a stone’s value and the price tag can rise by thousands of pounds for an even slightly heavier stone. However, the measure will never be used in isolation. A three-carat diamond will not be considered high quality unless it also scores well for the other three Cs.

Colour: hue, tone and saturation

For some gemstones, such as rubies and emeralds, colour can be the most significant indicator of rarity and quality. However, it is also a factor in valuing ‘colourless’ diamonds, because most have a slight hint of yellow, which diminishes their value.

Coloured gemstones generally have an ideal colour. The rarest and most desirable rubies are pure red rather than purplish-red. Slightly bluish-green emeralds and violet-blue sapphires tend to command the highest value. And, while they are usually thought of as colourless stones, coloured diamonds are often highly valued, with red diamonds being the rarest of all.

When assessing colour, gemologists look for three things:

  • Hue, which is the colour you see, such as red or blue.
  • Tone – the brightness or darkness of the gem.
  • The intensity of the colour, which is referred to as saturation. 

Clarity: the quest for perfection

Minerals and gemstones are formed over millions – and even billions – of years, deep in the Earth’s crust. During that time, crystals of other substances sometimes form within the stone. These are known as inclusions.

Experts grade precious gems for clarity by looking for these inclusions, which affect the stone’s brilliance. The fewer inclusions a stone has, the higher its value.

Flawless gemstones are extremely rare and most contain some blemishes or inclusions. However, they may be invisible to the naked eye. That means that while they might affect the stone’s value, they should not detract significantly from its beauty. 

Cut: creating the stone’s sparkle

It takes a highly skilled craftsman to turn a rough stone into a sparkling diamond. When a stone is cut, it refracts the light and its fire is revealed.

The cut is a crucial consideration in assessing a stone’s value, particularly in the case of colourless diamonds where brilliance is of paramount importance. However, it is probably the most difficult of the four Cs to measure. That is because the gemologist has to look at a number of factors, including the external and internal light reflection, scattering of light, fire and scintillation.

It is the quality, not the shape, of the cut that is used to value a precious stone. However, a gemologist will also assess the overall look of the stone, including its symmetry.

When you have selected your precious item, be sure you know how to store and care for it. And, of course, protect it for the future by taking out specialist jewellery insurance. That way you can focus on enjoying your unique gemstone.