The diamond price does not vary much and is not so dependent on market forces as say gold or silver.
Certain diamond producers, such as De Beers for example, do influence the market of course and maintain a storage of diamonds in order to keep the price up and stable.
The diamond price is more determined by the quality of the diamond. The cut of the diamond plays a big part. If the cut is wrong them this will adversely affect the brilliance and sparkle quality of the diamond. A properly Princess cut diamond, for example, will allow all the light entering the diamond to reflect back out through the top creating a tremendous sparkle and brilliance.
Unless it is a fancy specific colored diamond, the color of a diamond should be as close to clear or transparent as possible to allow the natural light to reflect through it cleanly.
Any taint will affect that light and produce a yellow or even ‘brown’ diamond and this is worth considerably less than a clear diamond of the same weight.
The clarity is important. Almost all diamonds have imperfections, called inclusions, and these inhibit the light from flowing through the diamond. A good diamond is one in which the inclusions are so small they cannot be seen with the naked eye.
Lastly, because the first three of the 4Cs are most important, the carat weight of the diamond comes last and really only determines its size by weight.
It is very important then that an independent appraisal of the diamond is done to show the qualities of the diamond. This then will help to determine its value.
The better the qualities of the 4Cs the more the diamond will be worth as compared to a diamond with lesser qualities.
This is why it is important to ensure you get an independent appraisal.
Keeping in mind the above points will assist you to know the diamond price and how much you should pay for it.