The name comes from Spanish, placera, meaning "alluvial sand." It means that mining the precious metal deposits (mainly gold and gemstones) found in alluvial deposits sediments of sand and gravel in modern or very old stream beds. The metal or gemstones, having been moved by torrent flow from a unique source like a vein, is characteristically only a minuscule part of the total deposit. The containing material might be so loose to securely mine by tunneling. Where water under pressure is obtainable, water under pressure might be used to mine, move, and separate the valued material from the deposit.
Placers supplied most of the gold for a hefty portion of the prehistoric world. Hydraulic mining techniques like hushing were used broadly by the Romans athwart their empire, but particularly in the gold fields of northern Spain after its take-over by Augustus in 25 BC. One of the main sites was at Las Medulas, where seven 30 mile lengthy aqueduct were used to work the alluvial gold deposits during the first century AD. In North America, placer mining was well-known in the context of more than some gold rushes, chiefly the California Gold Rush, the Fraser Canyon Gold Rush and the Klondike Gold Rush. Placer mining carries on in a lot of parts of the world as a basis of diamonds, industrial minerals and metals, gems (in Myanmar and Sri Lanka), platinum, and of gold (in the Yukon, Alaska and British Columbia).