Impure gold

Impure gold, as it commonly occurs in deposits, has
a density of 16 to 18, whereas the associated waste
rock (gangue) has a density of about 2.5. The difference
in density enables gold to be concentrated by
gravity and permits the separation of gold from
clay, silt, sand, and gravel by various agitating and
collecting devices such as the gold pan, rocker, and
sluicebox.
Mercury (quicksilver) has a chemical affinity for
gold. When mercury is added to gold-bearing material,
the two metals form an amalgam. Mercury is later
separated from amalgam by retorting. Extraction of
gold and other precious metals from their ores by
treatment with mercury is called amalgamation.

Recovering gold in a long sluicebox. Gold-bearing gravels are
shoveled into the upper end of the sluiceway; a series of riffles set
across the bottom of the box traps the gold. Myrtle Creek, Alaska,
circa 1900.