Buying Diamonds: What to Look for

Inside every diamond there is a fire. No other stone can split white light to create the magical play of coloured flashes that so captivates our imagination. Buying diamonds demands an understanding of the 'Four Cs': carat, clarity, colour and cut. Sculpting raw stones so that each seems to blaze and spark is the essence of the diamond-cutter's art. Some knowledge of it helps with buying diamonds.

A diamond's weight is measured in carats, with one carat equal to 200 milligrams. Smaller stones are measured in points (fractions of a carat). When buying diamonds, size isn't everything. A large stone with specks or cloudiness is worth less than a smaller diamond without flaws. The largest flawless cut diamond is South Africa's Centenary Diamond, discovered in 1986, weighing a massive 599 carats (just under 120 grams).

A diamond's clarity depends on the presence or absence of tiny flaws, visible when magnified ten times. Diamonds are compressed carbon. As they form, other crystals may be caught within. Sometimes the pressure causes tiny fractures in their internal crystal planes. Inclusions, minute cavities and cracks mar the stone's brightness. If you're buying diamonds rough, remember that surface blemishes affect value less than flaws of internal structure and can be excised. Depending on size and location, internal flaws not only affect brilliance but may make stones vulnerable to fracturing. Over three-quarters of the world's diamonds fail on clarity and are used in industry.

If you're buying diamonds, remember that clarity is also linked to colour. Though we think of diamonds as 'white', only diamonds with no impurities or crystalline deformities are truly colourless, and they are exceptionally rare. Some diamonds are valuable because they are coloured, but colour can also be a flaw. When buying diamonds a key caveat is that yellowness in white diamonds lowers value.

Diamonds may be yellow, blue, pink, green, orange and even red or black. Stones from the Kimberley area, South Africa, include nitrogen that sometimes gives the stone a bright yellow hue. These 'canary diamonds' are greatly prized. Blueness can be due to boron or hydrogen. Pinks and reds hues are determined by a gem's crystal structure, which affects the absorption and reflection of light. Buying diamonds is easier if you understand these variations. Famous coloured diamonds include the blue Hope Diamond and an Iranian pink diamond known as the Darya-I-Noor ('Sea of Light').

The diamond-cutter brings out the true sparkling potential of rough diamonds. A diamond's cut concerns proportion, not just shape (such as square or round). The diamond-cutter sculpts diamonds to eliminate flaws, faceting it to make the most of the unique arrangement of crystal planes in each. If you're buying diamonds, you're buying a work of art. The cutter's vision is key to releasing the light and fire within. The art has been honed over centuries with mathematical precision. New techniques such as laser cutting have perfected the process.

Diamond cuts optimize brilliance (the light it reflects) and a stone's ability to split and disperse light, creating those magical fiery flashes (scintillation). People buying diamonds often go for the best-known cut, the round brilliant. Its popularity is due to the fact that it is, as its name suggests, brilliant and sparkling, but also because it creates a strong stone. Other cuts, such as the teardrop (a variation on the round brilliant cut) are more fragile, affecting insurability. When buying diamonds people rarely consider this. Other variants of the brilliant include oval, heart-shaped and triangular cuts. A popular modified brilliant cut is the 'Princess', a square-faced stone.

Step cuts are another type, featuring a square or rectangular face faceted beneath With rounded corners, it is known as the emerald cut. Antique and art deco jewelry used step cuts extensively but are rarely considered by people buying diamonds. The rose cut is another antique cut now rare. Mixed cuts combine features of both brilliant and step cuts.

The lustre of a diamond refers to its surface reflectivity. Though less important than a stone's internal properties, low lustre affects the brilliance reflected from within. For the same reason, diamonds need to be kept free of dirty or oily residues. There's no point in buying diamonds then stopping them sparkle! Products on the market, including chemical cleaners. A popular alternative is the ultrasonic bath, which uses soundwaves to dislodge adhesions.

Our romance with diamonds predates the Middle Ages. Buying diamonds today is a matter of science too. Understanding the 'Four C's' is key to smart purchasing.