A diamond buying guide for first-time buyers

The four Cs in plain terms

• Cut: the only C that is purely human — a well-cut stone returns light, a badly cut one swallows it. Never compromise on cut.
• Colour: D is colourless, down the alphabet. G and H look white in most light and cost far less than D.
• Clarity: SI1 and above are clean to the naked eye. Buy clarity with your eyes, not the certificate.
• Carat: size is the last variable, and the one you feel most. Decide carat after the first three.

The certificate question

Ask for IGI or GIA certification and read it yourself — the certificate states cut, colour, clarity and carat, and carries a number you can check online.

A diamond's certificate is its passport. If a jeweller hesitates to show it, you already have your answer.

The question nobody asks

Ask where the diamond will be set — a good setting makes a modest stone brilliant, and a bad one hides a great stone. Any jeweller should be able to talk about this for twenty minutes.