Why can gravity concentration separate gold from sand and gravel?

Gravity concentration separates gold from sand and gravel because of the significant difference in their specific gravity. Gold is a dense metal, whereas the surrounding sand and gravel consist of much lighter materials, such as quartz or feldspar.12

In mineral processing, gravity techniques—such as jigging, tabling, or the use of a sluice box—exploit this density disparity. When a slurry of ore is passed through these systems, the heavier gold particles resist the flow of water and settle or are retained by mechanical features like riffles or grooves. Conversely, the lighter sand and gravel particles are easily carried away by the water current as tailings. This process effectively concentrates the valuable gold by allowing the less dense waste material to be washed away.3451