Diamonds are generally priced by carat which is the weight of a diamond. Factors such as the cut, shape, clarity and color influence the price of course and these factors can make a big difference. But the main criterion is the weight.
What many people do not understand is why the price of a diamond increases so much when the carat increases. The price per carat increases with the weight of a diamond. This is due to rarity. Small diamonds are much more plentiful than larger ones. In fact the larger the diamond the more rare it is. This affects the price per carat and, for example, a round one carat very good diamond might be anywhere between 5,000 and 10,000 dollars for the carat, a 3 carat diamond could fetch anywhere from 10,000 to 100,000 a carat. A 20 carat diamond can fetch millions even though the size is only 20 times as much
The other factors that affect the price are Shape, Cut, Colour, Clarity, Depth, Table, Symmetry and Polish. Each can add or subtract up to many thousands to the total cost of the diamond as well.
When it comes to diamond comparison then it is important to ensure all these factors are as close to being the same as possible. As each diamond is different and it one is very unlikely to find two exact diamonds this can be difficult. One has to employ some commonsense and do a fair amount of due diligence when it comes to understanding the price of diamonds.
The first step is to decide what sort of diamond you are looking for in terms of size and quality. Once that is established you can then set about looking at various diamond sites to see what sort of prices are being asked for each diamond that is as close to the diamond you are looking for as possible.
Some very good examples of diamond prices can be found at Live Diamond Prices and here you can scroll through around 14,500 diamonds to get a good idea of the price for each size and quality.
The more rare diamonds also have a potential investment value. The bigger a diamond the higher price it will command and many people buy larger diamonds, such as 10 carats and over, purely for investment purposes. Over a period of years some of the more famous diamonds have commanded successively higher and higher prices due to their rarity and subsequent fame. Ok if you have buckets of money I suppose. For most of us, however, it is a case of what we can afford to make our lives shine just that bit better.