Mineral deposits form when minerals concentrate within rocks. They become economically viable if the concentration of valuable metals or materials is high enough. The grade and concentration factor determine if a deposit can be mined profitably. Placer deposits of dense minerals like gold form in stream beds. Placer mining excavates these alluvial deposits using screens, jigs and sluices to separate minerals from rock. Veins of valuable metals like gold are often found within quartz deposits.