Diamonds are graded primarily by the 4 C’s. Carat Color Clarity and Cut. There are also other factors but these are the most important so when you get an appraisal, ask for these in addition to the value. A Round D Flawless is usually the most valuable. Knowledge is everything but rest assured, you will never know exactly how much your diamond is worth but evaluating it using the 4 C’s may prevent you from not getting completely ripped off.there are on line charts which will give you some idea once you know the 4 C’s. But unless you have a GIA Report, remember these factors are only as good as the person evaluating the diamond.
Diamond Calculator | Long's Jewelers
A side story. I knew Martin Rappaport in the early 1980’s when he started the Rappaport Diamond Report to try and standardize diamond valuations It was met with worldwide industry resistance and he actually received death threats. His report survived and is now used extensively worldwide. But it has so many nuances it is useless to the average consumer which is probably why he survived all these years. He is still criticized even by the people at the Debeers Syndicate who have close to a monopoly on diamond supply. The mid 1980’s was a treacherous time for the diamond industry and final,y succombed to the Greater Fool Theory. Diamonds were traded back and forth among brokers and dealers with ever rising prices not based on anything but the last sale. A D Flawless one carat quadrupled to $35,000 and higher but valuations collapsed a lot quicker than they rose and hundreds of millions were lost by banks who financed the dealers.
Martin Rapaport - Wikipedia