 By the late 1890s, dredging technology (also invented in California) had become economical, and it is estimated that more than 20 million ounces (620 t) were recovered by dredging (worth approximately US$28 billion at December 2010 prices).Both during the Gold Rush and in the decades that followed, gold-seekers also engaged in "hard-rock" mining, that is, extracting the gold directly from the rock that contained it (typically quartz), usually by digging and blasting to follow and remove veins of the gold-bearing quartz.By 1851, quartz mining had become the major industry of Coloma.