The metaphor, "A diamond in the rough," refers to something that could be made valuable or special with a little polishing. An actual diamond in the rough typically looks like a small, nondescript stone that most lay people would hardly notice and would not recognise as a precious gem. If you have a stone that you believe could be a diamond, there are certain characteristics to look for and also tests that can be done to help you determine whether you have a valuable gem or a worthless dud.
Research the area in which you found the stone you suspect of being a rough diamond. If the area is known for having diamonds, such as Arkansas's Crater of Diamonds State Park, then there is a possibility that you have stumbled on a diamond.
Study the stone's features. Most diamonds are small. A one-carat gem is approximately the same size as a green pea. Rough diamonds should have a lustre or sheen to their appearance, and they may also feel slightly oily to the touch.
Look into the stone. Most rough diamonds are translucent, so you should be able to see into a stone, but not through it, as if it were clear.
Compare the shape of your diamond against pictures of diamond roughs. Many rough diamonds have a distinctive geometric shape, while others, such as those found in the Crater of Diamonds State Park may appear almost round and smooth.
Place your rough diamond in water. Real stones will sink. Although some fake stones will also sink, many others will float to the surface.
Take your rough diamond to a jeweller or gemologist. An expert can perform several tests, including one that uses electrical conductivity, on your rough stone to see if it is in fact a diamond.