Here is a small video that show you how a mine in surface is working. This mine is located not to fare from Chanthaburi, Thailand. It is a Corundum mine, that mostly give green sapphire. The rough are extracted by density. The corundum density is around 3.80 to 4.05 depending what kind of element compose the stone, like Fe, Ti, Cr (That give the color beside the basic components that is (Al O)) that is more than the gravel and soil's density.
At the end of the day the owner come to check the run. People interested to buy have to buy all the run, you can not choose the stones or buy single rough.