The grading system for diamonds is quite complex when compared to items like coins.
You could have a large stone with bad color or cut that could be worth way less than a stone half its size with a better color and cut. It can be pretty confusing.
If you have a really big rock, let a jeweler YOU TRUST take a look at the stone first, then see if he recommends having it certified. After the stone has been certified you will then be able to track the market price on it. Just an idea.