How to buy a diamond

Cut is the only one of the four Cs that depends entirely on human skill rather than nature, and it does more for a diamond's appearance than the other three combined. It describes how precisely the facets are angled and proportioned, and therefore how much light the stone gathers, bends, and throws back at you.

A poorly cut diamond leaks light through its base. It can be flawless in clarity and colourless in grade and still look dull, because the light entering the top escapes rather than returning to the eye. A well cut stone of lower clarity will outshine it every time.

Grades run from Ideal down through Excellent, Very Good, Good and Fair. Ideal and Excellent are where you should be looking. The difference between them is subtle; the difference between Excellent and Good is not.

If your budget forces a compromise, make it somewhere else.

Clarity measures the natural marks formed inside a diamond as it grew — tiny crystals, feathers, and clouds that geologists call inclusions. Almost every diamond has them. They are evidence the stone is real.

The scale runs from Flawless at the top through IF, VVS1, VVS2, VS1, VS2, SI1, SI2, and down to the I grades. The important threshold is VS2. At VS2 and above, inclusions cannot be seen without magnification — the stone is what the trade calls eye clean.

Paying for VVS1 over VS2 buys you a better certificate, not a better looking diamond. The money is usually better spent on cut, or on a larger stone.

Below VS2, look at the stone's images carefully. Some SI1 diamonds are eye clean and represent real value. Others have inclusions sitting directly under the table where you will see them every day.

Colour is graded from D to Z, where D is entirely colourless and Z carries a visible yellow or brown tint. The scale sounds precise, but the eye is not. Most people cannot detect any warmth in a diamond graded H or above, particularly once it is set.

Where colour becomes visible depends on the setting. A stone set in white gold or platinum shows warmth more readily than the same stone in yellow gold, which masks it. If you are buying yellow gold, you can comfortably drop to J or K and save meaningfully.

Carat is a measure of weight, not size. Two stones of identical carat can look noticeably different depending on how they are cut — a shallow stone spreads wider and appears larger, but sacrifices brilliance to do it.

Prices jump sharply at the round numbers. A 0.90 carat diamond costs considerably less than a 1.00 carat of the same quality, and the difference in diameter is under half a millimetre. Buying just below a threshold is the single easiest saving available.

Lab grown diamonds are chemically, physically and optically identical to mined ones. They are not simulants — not cubic zirconia or moissanite, which are different materials entirely. A lab grown diamond is a diamond, graded on the same four Cs by the same laboratories.

The difference is origin and price. A lab grown stone typically costs a fraction of an equivalent natural one, which means a substantially larger or higher quality stone for the same budget.

The trade off is resale and rarity. Natural diamonds hold value in a way lab grown stones currently do not, and their scarcity is part of what many buyers are paying for. Lab grown prices have fallen steadily as production has scaled.

Neither choice is wrong. If size and quality matter most, lab grown gives you more. If permanence and rarity matter most, natural is what you are looking for.

Every diamond worth buying comes with an independent grading report from a laboratory that has never owned the stone. GIA and IGI are the two most widely trusted. A report from the seller, or from a lab nobody recognises, is not the same thing.

The report confirms the four Cs, but it also carries details that matter and are easy to miss. Polish and symmetry describe finishing quality — aim for Very Good or better on both. Fluorescence describes how the stone reacts under ultraviolet light; faint is harmless and occasionally beneficial, while strong fluorescence can make a diamond look slightly milky in daylight and is priced lower accordingly.

The report also includes a plotted diagram showing where the inclusions sit. This is worth a minute of your time. An inclusion near the edge, hidden by a prong, matters far less than one directly beneath the table.

If a seller cannot produce a certificate, walk away.