As hopeless romantics prepare for a Christmas proposal, Emily O'Sullivan has some advice on buying an engagement ring. The key? Remembering the four Cs

For most men, buying an engagement ring is a red-faced, sweaty-palmed experience. Diamonds are glorious, bewitching and fascinating, but they can also be intimidating, confusing and wildly expensive. Not only that: inscrutable terminology such as "fire", "clarity", "inclusions", "dispersion" and "scintillation" can make you feel trapped beneath an avalanche of rhetoric. But the leap from diamond dunce to diamond don isn't infeasible. Learn the lingo and you can't go wrong.

Diamonds have an unprecedented importance in Western culture thanks to our proclivity for ostentation. Ghetto- fabulous superstars emphasise the bling factor with layerings of oversized rocks, while Hollywood's hottest names drape themselves in show-stopping gems at premières and award ceremonies. This isn't altogether a recent phenomenon. The Greeks, Romans and Hindus were equally captivated by diamonds, believing them to impart invincibility and heighten sexual prowess. These talismanic qualities stayed with the diamond over the centuries, and, long before they became symbols of love, 14th-century French and English monarchs were showing off their sovereignty with lashings of sparkling stones. The world's largest high-quality cut diamond, Cullinan I, still resides in the head of Queen Elizabeth's sceptre, weighing in at a mind-blowing 530 carats.

Buying diamonds can be complicated, but the right approach always helps. At the heart of the matter are the four Cs - carat, colour, clarity and cut - and if you're going to part with an extraordinary amount of money, then you'd be well advised to do some homework. The four Cs allow both jewellers and consumers to ascertain whether a diamond can cut it or not. No surprises there, but what can often make the mind boggle is the way they are explained. The pyramid system charts diamonds in terms of rarity: those at the top of the pyramid are the rarest and most shockingly expensive. Those at the bottom aren't up to much.

The first C on the list is carat, or weight. A carat is divided into 100 points, so if you have a diamond that is a quarter, or .25, of a carat, it's described as a 25-pointer. Perhaps unsurprisingly, many of us are still hung up on size, and there's a peculiarly large jump in price between a .99-carat and a one-carat diamond.

"That's purely because people want to say they've a one-carat on their finger," says Gerard Appleby of Appleby jewellers in Dublin. "Actually, the difference between a .99 and a one-carat is very, very small, but the price differential can be 15 per cent or 20 per cent." The most popular weight is between half and one carat, especially for engagement rings.

Colour is the easiest C to fathom. Essentially, the whiter the diamond, the better its quality. "The darker diamond tends to absorb a lot of light instead of transmitting it," says Appleby. "The whiter ones fire it all out." If you're wondering what colour to go for, then keep in mind that the optimum colour grade is D: not very affordable, but the best.

It's when jewellers start to talk about clarity that people start to nod off. The most difficult to grasp of the four, clarity is basically the amount of inclusions, or "nature's fingerprints", left in the stone after cutting. Although clarity is definitely a consideration, it's probably the least important of the Cs for most people - and it can be assessed only by looking at a diamond through a microscope on a white background. Unless they're flawless, in which case you're probably not going to be able to afford one anyway, diamonds have imperfections, and as long as you go for one that's rated SI1 or above, you're onto a safe bet.

Although all the Cs should be considered fairly equally, cut is universally recognised as the most important. It refers to the angles and proportions created during the cutting process. "The cutting is what makes a diamond look alive," says Appleby. "It's also the part that man brings into the equation. Everything else is done by Mother Nature. The better the cut, the brighter the diamond and, therefore, the more desirable." Diamonds that are cut too deep and too shallow are less brilliant and therefore less valuable, but remember that the shape of your diamond - princess, round brilliant, emerald - does not affect the cut.

Lurking ominously behind the big four Cs is the hidden fifth C: cost. Diamonds rarely come cheap. According to most jewellers, the average amount spent on an engagement ring is between €2,000 and €3,000.

But should men still be forking out the traditional amount on a ring? "I don't know if the one or two months' salary thing still applies," says Declan Melody of Diamonds Direct in Mayo. "In some instances you have couples buying stones together. They know exactly what they want and what they're going to spend, as their finances are pooled. The whole thing has changed in the last 20 years."

Jacinta O'Brien, editor of Weddings - Irish Style, believes that men should try to establish the tastes of their future bride and act accordingly. "People should do what suits them as a couple, whether it's buying a simple band with an inscription that costs €60, or a rock that takes three years to save for."

Another C that is rarely considered at such a romantic time is conflict. But conflict diamonds is the term applied to diamonds that have been used to fuel conflict and civil war in countries such as Sierra Leone, Angola, Liberia and the Democratic Republic of Congo.

After mounting international pressure, the jewellery industry has cracked down on conflict diamonds, implementing a series of warranties or invoices that enable diamonds to be traced to their source. "If the oil business was a quarter as vigilant as the diamond industry," says Appleby, "the world would be a better place."

But the subject still occasionally erupts. Global Witness, an investigative organisation, issued a report in April this year illustrating how al-Qaeda used diamonds to finance its activities. Appleby has worked closely in the past with Global Witness, but he's hesitant about the connection. "The jewellery business worldwide is still largely a Jewish business," he says. "It would make me doubt the whole veracity of the story about al-Qaeda, because I cannot see Jewish diamond traders trading with people of that persuasion."

With tighter controls in place, it's highly unlikely consumers would ever come into contact with conflict diamonds. If you're still concerned, ask your jewellers what they are doing to ensure their diamonds are not illicit.

What to watch out for

A grading certificate Essentially, a diamond passport. It should come from the Diamond High Council in Antwerp or the Gemological Institute of America. It details the diamond's cut, colour, clarity, weight and shape. "If you have your certificate nobody can argue with the quality of your diamond," says Paul Sheeran of Paul Sheeran Jewellers. Ensure a reputable grading certificate is available before buying.

Fire Can loosely be described as how a diamond handles light; largely a characteristic of a brilliantly cut gem. Scintillation is a diamond's sparkle; dispersion is its distribution of light.

Lasering Although unethical, diamonds are occasionally lasered to remove marks or inclusions. If a diamond has been lasered it should be noted on the grading certificate.

Fluorescence A quality found in about 10 per cent of diamonds. Used to describe a blue-white effect.

Light "Daylight is the ideal light," says Neville McDowell of Weir & Sons in Dublin, "but you can't sell diamonds at the side of the street." When viewing diamonds indoors, make sure the light is as similar to daylight as possible.

Shape Mayo jeweller Declan Melody advises people to buy a single stone in a round brilliant. Optically, the round brilliant is believed to be the most perfect. The princess comes a very close second in terms of quality and popularity. Other popular options are emerald, oval and pear.

Buying abroad Jewellers says there isn't much of a price difference between diamonds bought in Ireland and abroad, but some people like to find out for themselves. When buying abroad, insist on a grading certificate and visit a jeweller recommended by friends or colleagues. And keep your wits about you. "You pay for quality," says Sheeran. "If a deal looks too good to be true on a diamond, it probably is."