Gems undergo a lot of changes once they've been cut, shaped and polished by professionals. The raw gems that a person finds in a mine or laying around on the ground have different characteristics that make them stand out. This is especially true for diamonds, which are much different in their raw form than they are once they've been cut, polished and set in a diamond engagement ring.
Look for the color of diamonds found in that area. In Crater of Diamonds State Park for instance, the raw diamonds are white, yellow and brown, in that order of frequency.
Examine the size. Most diamonds that are found aren't terribly large. They're perhaps 20-25 points (and there are 100 points in a carat). This makes them roughly the size of a paper match head.
Feel the stone. Raw diamonds have a slick, almost oily feel to them.
Check the remaining qualities of raw diamonds. Raw diamonds are translucent, so you can see into them but not through them. These stones are also lustrous like steel or lead.
If you think that you've found a diamond and it meets all the prerequisites, ask a jeweler to test it.