Hard rock or raw pit run [stream bed] has to be cracked to 20 mesh in order to burn off the sulfides/sulfur then the gold drops out as free gold Gravel is just small pieces of hard rock ~~~~all goes through a jaw crusher to 20 mesh [or ball/rod mill] Standard wash plant will not release the free gold trapped in the rocks ~~~~~ out of the sluice they go.PM me your e-mail for info if you want claim info , hope this helps , NEWMONT/KINROSS are now turning to AUTOCLAVE , thanks ANDY/COPYTELE