Instructions
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.