With a big Royal Wedding just happened, many men may be pondering whether it's the right time to pop the question. If you decide to go down on bended knee, one of the many things you should be thinking about is the ring. Women may say they're not bothered about the size of the rock in public, but in private, you know they're going to be showing it off to their cooing friends -- so you need to get it right.

Buying jewellery can be a daunting enough task as it is, but buying diamonds take this to another level. Samuel March, diamond specialist has been a Hatton Garden diamond dealer for 15 years. He says that, despite marriage rates slipping, there will always be a strong trade in precious gems, "Diamonds are a special and expensive item that you give to your girlfriend to prove your commitment to a long-term relationship, even if you don’t want to get married. People might not be buying engagement rings, but they might buy eternity rings, or a nice pendant that shows that they care."

We spoke to Samuel about what things every man should consider when buying diamond jewellery.

There are quite a few companies that are still experimenting with diamond cuts, but the main shapes are: the round brilliant cut, the princess cut (square), the emerald cut shape (rectangular), the marquee (oval), the trillion (three sides) and the usher cut, which is like an emerald cut, but more squared.

No.10 Trends in diamonds

A lot of people tend to go for round diamonds, although the princess cut is the most popular for engagement rings.

If your lady has long fingers you may want to go for the marquee cut, as it has a long oval shape.

No.9 Colour

When you put a nice beautiful diamond on her finger, the first thing that people will notice is the colour and the shine of the diamond, so obviously the whiter the stone is, the better it will look.

With colour we grade it from the letter D for diamond (it’s basically colourless and the whitest) all the way down to M, which is where it becomes dark yellow. People go for the brightest that they can as it is rarer.

To the naked eye, there's no difference in colour between any diamond graded from D to F, so the biggest sellers are those graded F or G, because the price is much cheaper than the D colour, but they still look white.

No.8 Clarity

Little crystals that form inside diamonds are called inclusions. They are natural marks, because the diamonds themselves are made purely of carbon which can be black, or purely clean, so the cleaner diamonds are more expensive, and they are rarer. When a woman has a beautiful ring on her finger and she walks around, people will want to look at the stone, so you don’t want a diamond where you can see the inclusions.

The very best stones are internally flawless, so there are no visible marks when you look with a loupe (eyeglass). There are a few grades of clarity which contain marks that you can see with the naked eye.

No.7 Cut Grade

The cut grade is quite an important factor (especially in a round stone) because the better the stone is cut, the more light it will have.

Even if you don’t know much about diamonds, it's clear that a stone with a nice cut gives you a lot of light, so you will think that stone is really good in comparison to others.

No.6 Carat

Simply put, this is about the weight of a diamond and it's a big factor in the budget when you’re looking for an engagement ring. The bigger the stone is, the more you'll pay for it; the smaller the carat the less you pay. You're actually paying for the weight of the rough stone, which your diamond came from before it was cut. If you’re going to get a one carat diamond, the rough diamond will weigh about two carats, so when it’s polished you lose about 50%.

If you desperately want a one carat stone, but can’t afford it, you can sacrifice one of the other Cs (colour, clarity, cut grade).

No.5 Conflict diamonds

Diamonds should come with certification all of the time, and you should show that they’re conflict-free. When diamonds are bought for trade they should use the Kimberley Process, which is a certificate that all diamond dealers must use to show that the diamonds come from a legitimate source, and can be traced back.

There's currently no way by looking at the diamond itself, to tell whether a diamond has come from a legitimate source: but in the Chinese market, they’re actually working on creating something called the 'Forever Mark'. It’s a tiny mark on the stone that doesn’t affect the colour, cut or clarity of the stone, and it’ll act as a passport for the stone to show that it came from a legitimate source.

No.4 How can you trust the seller?

If you’re buying a diamond it will have a certificate which guarantees the specification of the stone. This certificate will be from either: the International Gemmological Institute, the American Gemmological Institute, the Gemmological Institute of Antwerp or HRD Antwerp. If there’s no certificate, don’t buy it.

No.3 Big is beautiful

The bigger a diamond is, the more important it is, because it’s rarer to find. There are not many ten to six carats around and so the value of the stone will hold, and most probably go up.

When you buy something big and beautiful, it will be actually be an investment for the future. If you buy online you can find a decent discount on something bigger too.

No.2 You don’t have to break the bank

If you’re buying a diamond from a diamond dealer who has good connections and who knows about the right colour, and carat, he will get you the best price. If he doesn’t know what he’s talking about, then the person selling diamonds to him will be ripping him off, and it’ll more expensive for you.

So always buy from a good company that has been around for a while, and that has experts on site; you need someone who is on hand, who can speak to the customers. The more open they are, the more trustworthy they are.

No.1 Diamonds are forever

The youngest diamonds are millions of years old; they’ve been crystallised over many years, and are made only from carbon.

So when you're buying diamonds for a woman remember that they are a lifetime investment because they are the strongest substance around and they really do last forever.