When a jeweller is assessing the value of a gemstone they break down their analysis into four key characteristics. These “Four C’s” allow a standardised scale to be applied to the evaluation process, which will place the stone on a scale of potential price.
The four C’s are Cut, Carat, Clarity and Colour.
Many varied cuts exist, and each is assessed on its own merit. The skill with which the lapidary has cut the rough stone is particularly important in antique jewellery, as without the aid of modern machinery and industrial science it was far more difficult to achieve mathematical perfection in the cutting of facets. Whilst lack of symmetry can be difficult to see with the naked eye it can be identified by a skilled observer using a jewellers loupe, and is one of the main indicators of the age of a stone.
To read more about the different types of gemstone cuts and their respective properties take a look at our helpful guide here.
The weight of a gemstone is measured in carats, with one carat being equal to one fifth of a gram. The word originated with the Greek word keration meaning ‘Fruit of the Carob’, and was a reference to the elongated seed pod of the carob, an evergreen shrub. The word later came to be used as a unit of measurement by early jewel traders, and has remained in use to this day.
This weight measurement is not the same as carat or karat when applied to gold, which is a measure of how much pure gold (24 carat gold) is present in an alloy. As pure gold is extremely soft it is almost always mixed with other metals to increase its durability when formed into jewellery.
Gemstone carats are metric, and divided into hundredths, so a stone can be measured anywhere from 0.01 carat through to the largest diamond ever faceted, which is the Golden Jubilee Diamond, weighing in at 545.67 carats.
Because smaller gemstones are found more often than larger specimens the price of larger carat diamonds exponentially increases as they increase in size. It is this rarity which drives the price of the largest stones into the millions of pounds.
Clarity is a measure of the purity of a gemstone, determining the amount and severity of inclusions, flaws and faults within a given stone. Very few gems of any type are completely free from flaws and inclusions. No matter how skilfully they have been cut a natural stone will likely have some marks within it, and this is more pronounced in certain types of gems. Whilst diamonds are naturally clearer, other extremely valuable precious stones such as emeralds routinely contain large numbers of flaws.
Although desirability, and price, increase with a higher amount of clarity a perfectly clear stone is so rare that it may be a sign that the stone is lab grown, or crafted from inferior materials to resemble a more expensive stone.
The scale used for diamonds is shown below. The various categorisations are assigned by an experienced observer using a microscope of jewellers loupe.
Included (I1, I2 and I3)
Included Diamonds contain flaws or inclusions that are obvious to the naked eye, and do not require any magnification or specific expertise to detect. Such stones contain numerous or severe imperfections that may be pronounced enough that they render the stone weaker and more prone to damage.
Slight Inclusion (SI1 and SI2)
These stones have flaws or inclusions which are usually not visible to the naked eye, but which become obvious when viewed under magnification. Assessing the severity of slight inclusions requires a very specific skillset as well as the correct tools.
Very Slightly Included (VS1 and VS2)
These diamonds have flaws that are small enough that they are difficult to observe with 10x magnification, and may require more powerful magnification to see. Specialist training or a great deal of experience may also be necessary to accurately classify this type of stone.
Very Very Slightly Included (VVS1 and VVS2)
The flaws in these diamonds are so small and slight that they require powerful microscopes and extremely specialised training partnered with experience to make a correct determination.
To meet the criteria a diamond must be free from any internal inclusions and external flaws. This degree of perfection is extremely rare, and very few natural diamonds are truly flawless. To make this determination it is often necessary to have the stone tested at a specialist laboratory with trained personnel and the latest technology.
To read more about how we assess and test our diamonds take a look at our in-house guide here.
Colour is most important when assessing stones that are not diamonds. The hue of green which an emerald exhibits, for example, can have a huge effect on its value and desirability. This is even more important when it comes to rubies and sapphires. Both of these precious stones are varieties of the same base mineral, corundum, and it is possible to find pink sapphires and pink rubies which are indistinguishable to the naked eye, yet can have profound implications with cost. Rubies are more scarce, and thus more valuable.
Diamonds are most often colourless, but there are varieties of diamond with chemical irregularities that cause them to hold a particular colour. Natural coloured diamonds, sometimes known as “fancy” diamonds, are incredibly rare and extremely valuable. The most common colouration is brown or yellow, which is often referred to as “champagne”. The colour scale above shows the main labels which are applied to this scale:
Diamonds with much more pronounced colour variations do exist. These can be the bright red of a ruby or the deep blue of a sapphire, with myriad other variations. These brightly coloured natural stones are some of the rarest materials on earth and command immense prices.
Colour testing of these pieces often includes techniques to determine where treatments have been applied to a stone to make it take on characteristics that it did not have originally. Heat, radiation, corrosive chemicals and pressure can all be applied by a skilled but unscrupulous jeweller to increase the value of their diamonds by forcing them to take on a colour. Artificial diamonds may even be lab-grown in a range of colours.
At the other end of the spectrum, particularly low-level diamond dealers may apply coloured dyes to a gem to try and imitate more valuable characteristics. These techniques are easily detected.
To find out more about the way in which diamonds are assessed take a look at our advice centre, which contains a number of helpful articles.