The nickname for Nevada is The Silver State, dating from the Nevada silver rush days of the mid 1800's. At that time silver was literally shovelled off the ground in Nevada. Heavy gray crusts of silver had formed on the surface of the desert over millions of years and were polished by dust and wind to the dull luster of a cow horn (called "horn silver").

A big silver bed could be tens of meters wide and more than a kilometer long (worth $27,000 a ton in 1860's dollars). The territory of Nevada and surrounding states were picked clean of silver within a few decades.

These "surface bonanzas" lasted only a few seasons, long enough to put up saloons and little else. The rough, violent life of many western movies reached its purest state in the Nevada silver camps.

Source:

Geology - Keep Watching the Ground: About.com

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State Nicknames - 50 States