Decide your budget - but be prepared for your fiancee to fall in love with something that might cost something north of your choice!
However, there is a myth that you must spend at least one month's salary but, really, you should spend an amount that you can comfortable afford. But, keep in mind that this is the most important piece of jewellery you're going to buy, apart from the wedding ring. It's meant to last a life time so try to focus on the cost-per-wear and ease the pain!
Does she normally wear jewellery that's delicate and feminine? If so, consider something classic and traditional - may be vintage.
On the other hand, if she normally sports chunky, statement pieces then a contemporary engagement ring might be a better choice. Andrew Geoghegan, an award winning designer of the most unusual engagement rings around, springs to mind in this case.
Only if the engagement ring is going to be a surprise. Assuming you're wanting to surprise your fiancee-to-be, then it is useful to have some idea. "Borrow" one of her rings and take it with you to give the jeweller a guide. Check that the ring you choose can be re-sized later if need be.
Rings like the one on the right are more difficult to resize.
Diamonds come in all shapes and sizes. The most popular are round brilliant cut diamonds but other cuts and shapes include emerald or square cut, oval, marquise, pear-shaped, and even heart-shaped. Diamonds such as emerald or square cut have fewer facets therefore any inclusions will be more obvious in lower clarity grades.
Then there is a choice of setting: rub-over, collar, tension, or claw set? Traditional styles can often look better in a claw set whilst contemporary designs can look great with a visible band of metal holding the stone. Tension, as the name suggests, is simply an open ring with the tension of the band holding the stone in place.
A surprise proposal is a very romantic idea and, if you're sure that she'll love whatever you choose, or you know exactly what she wants, that's great. However, if you'd be happier with a compromise, give some thought to buying a loose diamond and have it set after the proposal. It's the perfect compromise between keeping an element of surprise and ensuring your fiancee gets the ring of her dreams.
It's the worst case scenario; your fiancee doesn't like her engagement ring! Not to worry, we'll exchange the one you bought for another. You bought a loose diamond and it's not quite the one - we'll swap it. You had a ring designed and it's not quite right - a little more difficult (and expensive) but not impossible - it can always be remade.