Shape

There are eight common shapes for diamonds - round, princess (square-shaped), oval, marquis (shape resembles the hull of a sailboat), pear, emerald (rectangular), trillion (triangular) and heart.

Round, emerald and princess are the three most popular.

Modern cutting technology has resulted in more complex shapes, such as stars and butterflies, but do note that these sharp-edged fancy shapes are vulnerable to breakage.

Cut

This is commonly mistaken for the shape of a gem. It actually refers to the polish, symmetry, proportion and number of facets. A facet refers to a flat, polished surface on a gemstone.

With more facets, more light can bounce off a precious stone and enhance its brilliance. The most popular diamond cut is the brilliant, which is a round-shaped diamond with 58 facets.

Tanja Sadow, the dean of the Jewellery Design & Management International School in Bencoolen Street, says there are over 400 trademarked or patented variations of the traditional brilliant style today, 'some with as few as 30 facets and some with as many as 221 facets'.

The polish describes the smoothness of the diamond's facets and symmetry refers to their alignment.

With poor polish by jewellers, the surface of a facet lacks sparkle.

With poor symmetry, light is misdirected as it enters and exits the stone.

Tip: While all four Cs - cut, clarity, colour and carat - influence the price, experts say cut is the most important and has the biggest impact on the value.

The cut and the number of facets should precede the other 3Cs, says Julie Barry, the marketing director of American diamond specialist, Hearts On Fire.

Colour

Plain-coloured diamonds are graded by a scale that begins with D and ends with Z, depending on the degree of yellowish tint.

Colourless diamonds are graded D to F; near colourless diamonds are graded G to J and diamonds in the rest of the grades have a yellow tint.

The most valuable plain-coloured diamonds are graded D, E and F.

Tip: If you are on a budget, buy grade F instead of D or E and J instead of G or H.

For example, a J colour is often 15to 20 per cent less expensive than a diamond graded G to H in colour, yet they are all nearly colourless to the eye.

The savings on colour will allow you to buy a larger diamond for the same price.

Diamonds come in a variety of colours - from grey, blue, yellow, orange, red, green and pink to purple, brown and black. While pure diamonds are transparent and colourless, coloured diamonds contain impurities or structural defects.

Yellow and brown diamonds develop their colour due to the presence of nitrogen impurities.

They typically cost less than colourless diamonds as they are more commonly found, says Gary Goh, the brand manager of local jewellery chain Goldheart.

Diamonds which are light blue contain scattered impurities within the crystal matrix and green diamonds were bombarded by nuclear rays during growth.

Coloured diamonds are valued based on their rarity and intensity and are not necessarily worth less than colourless dazzlers.

'Pink, blue, purple and orange diamonds appear less frequently in nature and are thus more precious than colourless diamonds,' says Goh.

'Green and red are the most prized as they are extremely rare.'

Clarity

Clarity relates to the appearance of a diamond's internal defects, called inclusions, and surface defects called blemishes. Buy a rock that is as flaw-free as possible or it may be prone to cracks, says Goh.

The Gemological Institute of America, the world's largest non-profit institute in gem research, grades diamond clarity by six categories - flawless (FL), internally flawless (IF), very very slightly included (VVS), very slightly included (VS), slightly included (SI) and included (I).

Tip: Buy at least a VS if you cannot afford a flawless stone. Though these have inclusions, they are not easily noticeable to the naked eye. Unless someone examines your diamond under magnification, he will not be able to spot the flaws, says Sadow.

Carat

In diamond terms, carat refers to the weight of the stone. As with gold, the more the carat, the greater the stone's value.

Top rocks of 10 carats and above are rare and certain to appreciate over time.

Tip: Opt for 'nearly-there' sizes such as a 0.9 carat instead of 1 carat. It is likely to pass off as the latter to the naked eye but will cost about 30 per cent less, says Sadow.