Placer mining is the extraction of minerals from alluvial deposits, often at the site of flowing water. Minerals that have been eroded from an upstream source and carried by a river, called placer, are deposited along the course of the water and buried in sediment to be later extracted with minimal effort.
Gold is commonly found in placer deposits, and placer mining was a principle extraction method during California's gold rush of 1849. A typical placer mining operation involves scooping alluvial silt, either from the active riverbed or from the older deposits along its banks, and sifting the soil in water drawn directly from the river. Putting the soil in temporary suspension allows gold, and other heavy metals such as silver, to precipitate out to the bottom of the pan. Some gemstones, such as garnets and diamonds, also precipitate in this way, and collecting them can be done either by gradually pouring water and sediment off the top of the pan or overturning the pan and digging through the large grains at the top of the pile of silt.
Placer deposits typically represent only a small fraction of the total lode. For this reason, many placer miners work their way upstream toward the presumed source of the placer.