Black sand is sand that is black in colour. One type of black sand is a heavy, glossy, partly magnetic mixture of usually fine sands, found as part of a placer deposit. Another type of black sand, found on beaches near a volcano, consists of tiny fragments of lava.

While some beaches are predominantly made of black sand, other colour beaches (e.g. gold and white) can often have deposits of black sand too, pa rticularly after storms. Larger waves can sort out sand grains leaving deposits of heavy minerals visible on the surface of erosion scarps.

Black sand is used by miners and prospectors to indicate the presence of a placer formation. Placer mining activities produce a concentrate that is composed mostly of black sand. Black sand concentrates often contain additional valuables other than precious metals. Rare earth elements, thorium, titanium, tungsten, zirconium and others are often fractionated during igneous processes into a common mineral suite that becomes black sand after weathering and erosion.

Several gemstones, such as garnet, topaz, ruby, sapphire and diamond are found in placers and in the course of placer mining sands of these gems are found in black sand and concentrates. Purple or ruby-coloured garnet sand often forms a showy surface dressing on ocean beach placers.