The size of the diamond is called it's carat weight and actually refers to its mass. All other characteristics being equal, the carat weight is the biggest determinant of the price of a stone.
A one carat diamond ring will always cost more than a ring made of smaller diamonds that weighs the same or even more.
Generally, once you have decided on which cut, color and clarity you want, the next important decision is to decide on the carat weight.
The chart below exemplifies how different carat weight looks after it is set in a ring.
Tips to help you determine the right carat weight
Some considerations you will have to keep in mind: the size of the recipient's finger, any personal preferences, the type of setting and the most important factor, the budget. Try looking at the purchase in the following order.
First, decide on to your budget and then explore the various qualities and designs available in that price range.
Factor in the shape of the finger. If the fingers are long, a small carat diamond may look even smaller and vice versa.
Be sure to match the setting of the diamond ring with the carat size. For example, a wide band can make a stone look smaller than it might with a narrower band. Keep in mind the word "misfit" when matching diamonds with settings and use your own judgment. Here's one more thought regarding setting: choose a setting that gives maximum protection to the diamond.
The lifestyle of the person is also something to keep in mind. Someone who is very active will potentially subject a diamond ring to more knocks and scratches than a more sedentary person, particularly if the setting makes the stone stick out a lot.
Perhaps the only immeasurable consideration is personal preference. That's because many people don't have any regarding diamonds - "any diamond is a good diamond" - and it can be difficult to find them out without letting the person know you are shopping for one. But if someone says that she really likes champagne diamonds because they really look nice with her skin tone, then that should be a good indicator of how to proceed.