Posted on in Sustainability

Evaluation criteria for gemstones

The valuation of gemstones - and therefore their value - can seem obscure, with stones at any price, from quartz sold for a few euros to the Pink Legacy, a pink diamond that went for over $50 million at Christie's in 2018. Even within a single variety of stone, the value can differ greatly. So why is one stone worth more than another?


The difference in price between a quartz and a diamond can be easily explained by the rarity of the stones: quartz is much more common than diamond, and can be mined very easily. Thus, rarer stones are generally more expensive, especially if they are sought after. In addition, colored stones, like diamonds, are valued according to four criteria, the famous 4C's: carat, color, cut, clarity (in French: poids, couleur, taille et clarté). It is these criteria, along with a few other factors, that will determine that one ruby is more valuable than another.

Weight is the most fundamental data. It is measured in carats, one carat being one fifth of a gram. Generally, the larger a stone is, the more its value increases. But this increase depends on the stone: a 3-carat Paraiba tourmaline from Brazil will be considered a large stone, while a citrine of the same weight will not. In addition, the price per carat of a stone increases with its size: the price per carat of a ten-carat sapphire will be much higher than that of a one-carat sapphire.

Second criterion: color. To evaluate the color of a stone, several factors are considered: the color as such (red, blue...), its intensity and its distribution. Thus, a bright and uniform color throughout the stone, without any secondary tint, will often have a very high value.


The amethyst on the left has a strong color zoning


When we evaluate clarity, we're sort of evaluating the purity of the stone, whether it has inclusions or not. These inclusions can be minerals or liquids, which get trapped in the stone during its formation. While they may be sought after in specific stones, they are often seen as impurities that diminish the value of the stone. Some stones are more included than others: emeralds without inclusions are rare, while aquamarines or tourmalines without inclusions are more common. Clarity is assessed with the naked eye, except in the case of diamonds, for which a 10x magnifying glass is used.

Two types of cuts exist: faceted and cabochon cuts. Transparent stones with few inclusions are often faceted, while opaque stones are more likely to be cut in cabochon. To evaluate the size of a stone, one looks at its proportions, whether its facets are symmetrical, as well as the quality of polishing and the presence of external damage.

The evaluation of transparent diamonds is mostly based on the 4Cs. However, for colored stones, several other factors may come into play. The first of these is rarity. Geographic origin has, for some stones, a considerable impact. Stones from a particular locality are highly sought after: alexandrites from Russia, sapphires from Kashmir, rubies from Myanmar, paraiba tourmalines from Brazil, emeralds from the Muso mine in Colombia...

Kashmir sapphires are a good example: with their exceptional blue color, these stones are particularly sought after. However, mining in the region stopped decades ago. As a result, Kashmir sapphires have become extremely rare, so that their value is continually increasing. For example, a ring with an unheated Kashmiri blue sapphire of more than 35 carats sold at auction at Christie's for more than 6.5 million euros in 2015.

Many other factors can influence the price of a gemstone, but the most important is the taste of each individual: when someone likes a stone very much, he can pay a higher price. Stones are fascinating, a stone can be beautiful for one person and not for another; the most important thing is to find the stone you like.

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