Guide to buying a diamond

There are many traps for the unwary, such as bogus sales, "blood diamonds", and overpricing.
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  • Updated:26 May 2008

02.The four Cs

The value of a diamond is determined by the four Cs:

  • Carat One carat equals 0.2g and is subdivided into 100 points. However, small weight differences undetectable to the eye (such as between 0.99 carat and 1 carat or 0.74 and 0.75 carat) can make a large difference to the stone’s price. Weight isn’t the only factor, though. Depending on its quality, a smaller high-quality diamond may be worth more than a larger one of lesser quality.
  • Cut Diamonds are available in all kinds of cuts — brilliant, marquise and emerald are just three. Look at examples and decide which you prefer. The better the standard of the cut and quality of the finish, the better the brilliance and fire (brightness and sparkle) of the diamond.
  • Clarity Diamonds were formed millions of years ago through pressure and heat, and most contain imperfections or foreign material such as other stones or minerals, called inclusions. A diamond's clarity grading tells you if inclusions are present. There are different grading systems, usually ranging from flawless (loupe-clear) to a small inclusion only seen through a 10x loupe (VS1), right through to visible with the naked eye (I3 or P3).
  • Colour A perfect diamond is colourless, but most have a hint of yellow or even brown, which results in a lower grading ranging from D (exceptional white+) down to Z (tinted, usually yellow). However, very rare pink diamonds (mainly from the Argyle mine in WA) and 'fancy coloured' diamonds, such as intense yellow, can be very valuable.

Diamonds can be treated to improve their appearance. Treatments include artificial colouring, laser drilling to remove inclusions and fracture filling to conceal cracks. When a diamond is treated it’s considered less rare and is viewed as an inferior product to a natural untreated stone of similar specifications. Consequently it’s much less valuable.

Synthetic and even imitation diamonds are available as well — some are just as beautiful but much less valuable.

The way to make sure you’re getting what you paid for is certification — sometimes called the fifth C. A grading certificate from an independent laboratory exactly describes the characteristics of the stone.

There are independent Australian laboratories, some of them recognised by the World Jewellery Confederation (CIBJO). The best-known labs are GIA in the US and HRD in Europe.

Certification is usually done prior to setting the stone, so if the diamond is already mounted, it’ll need to be taken out.


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