There are hard-rock underground mines, and there are soft-rock underground mines. Coal deposits, for instance, live in relatively soft sedimentary rock. Gold deposits live in igneous or metamorphic rock, which is relatively hard, as do diamonds, copper, silver, nickel and zinc [source: Great Mining ...
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Underground hard rock mining refers to various underground mining techniques used to excavate hard minerals, usually those containing metals such as ore containing gold, silver, iron, copper, zinc, nickel, tin and lead, but also involves using the same techniques for excavating ores of gems such as diamonds.
Since the costs can be high in the exploration and removal of gold from the hard rock mines, large companies are created in order to raise the money necessary for the development of the mines, rather than the solitary individual or small group associated with placer mining.