The metal in a five-cent coin is named after a devil. German miners gave it that name because the ore cheated them.
In the Ore Mountains of Saxony, miners kept finding an ore that looked like copper ore. When they smelted it, no copper came out. Worse, the ore was laced with arsenic, and the miners who worked it often fell ill. The miners blamed a malicious spirit and called the ore Kupfernickel.
Kupfer is "copper." Nickel was a demon's name, a short form of Nicholas, as in "Old Nick" for the devil. Sources translate the compound in slightly different ways: "copper goblin" in Etymonline, "devil's copper" in the Elementymology reference, and "Old Nick's copper" in CIM Magazine. All point to the same idea, a fake copper planted by something wicked.
The word is old. The Elementymology reference cites a Schneeberg miners' register entry from 1654. It reached print in 1694, in a work by Urban Hjärne on useless ores.
The chemistry came from Sweden. In 1751 Axel Fredrik Cronstedt tried to get copper out of an ore and got a new metal instead. He later found that it matched the metallic part of Kupfernickel samples sent from Germany. In 1754 he named the metal by shortening the miners' word to Nickel. Etymonline describes the Swedish kopparnickel as a half-translation of the German. Torbern Bergman confirmed Cronstedt's results in 1775.
Nickel has a close cousin in the mines. Cobalt, which Etymonline says closely resembles nickel, is named after Kobold, a German household goblin. Miners blamed both metals on spirits for the same reasons: worthless-looking ore and arsenic fumes.
The coin name is much younger. American and Canadian five-cent pieces are called nickels because they are made of a copper-nickel alloy. The US nickel was introduced in 1866 and the Canadian one in 1922. So a pocketful of change still carries the name of a Saxon miners' demon.