Coarse Silicon Carbide tumbling abrasive. Inexpensive and excellent as a first step.

Use this on sharp and broken rock to smooth and round the rough edges. We suggest using 3 - 4 level tablespoons for three pounds of rough rock.
 

 WARNING: This product can expose you to Respirable Crystalline Silica which is known to the State of California to cause cancer. For more information go to www.P65Warnings.ca.gov