What Makes A Diamond So Valuable?

 What Makes A Diamond So Valuable?


Raphaelle Godfrey, Head of Jewellery at Chiswick Auctions is a qualified gemmologist. In this story, she provides us with a guide as to why diamonds are so valuable.




Natural diamonds are between 1 and 3.5 billion years old and are formed in the earth’s mantle under the extreme temperature and pressure conditions (approximately 1000 °C and 50 kilobars) that are prevalent about 150km below the Earth’s surface.  Diamonds are mainly composed of carbon, and chemically have the same composition as graphite (as seen in your humble pencil). However, it is their structure that make diamonds so special. The crystal structure makes diamonds much harder than graphite. As a result, in Mohs scale, diamonds have a hardness of 10 compared to graphite’s 1. In layman’s terms, only a diamond can scratch another diamond, whereas graphite can be scratched by any harder mineral.

In addition, diamonds are difficult to extract. The mining process – whether pipe, alluvial or maritime – often requires complicated machinery.  Once extracted, a thorough and complicated sorting process follows. Only 20% of the diamonds mined are of jewellery quality; the other 80% are allocated for industrial use.




While the value of diamonds often seem opaque, their pricing is fundamentally based on their specifications including the traditional ‘Four C’s’:

Carat – the weight: one carat (ct) equals 0.2g

Colour – the colourless ‘white’ (graded from D to Z) or ‘fancy’ colour of a diamond

Cut – the way the stone is cut to best bring out the sparkle and light of each particular gem

Clarity – the birthmarks within the stone itself, better known as inclusions

There is a fifth C, which also has an impact on the desirability of a diamond:

Certification – the certificate given by a gemmological institution about the specifications of a diamond


As far as the value of a diamond is concerned, we should always remember that diamond pricing isn’t linear but exponential and that very small differences in a diamond can have a huge impact on its value. For these reasons, think through which specifications are most important to you, in order to make the best choice for your budget.




If you have questions about a diamond you may wish to buy – or sell – just ask our jewellery specialist, Raphaëlle Godfrey. We will also be running a diamond masterclass during our Spring season of sales. In this informal 2-hour session, participants will be guided through many of the aspects covered in this story, and get to see and handle a range of diamonds. To register your interest, please email Raphaelle.