1876 in reference to currency, "consisting of but one metal; comprising coins that consist of either gold or silver, but not both," from mono- "single" + metallic. Opposed tobimetallic. In chemistry, from 1861.
early 15c., "of metal, made from metal," from Latin metallicus "of or belonging to metal," from Greek metallikos, from metallon "metal, ore" (see metal). Specific use in chemistry, indicating the condition of a metal in which it exists by itself, not mineralized or combined with substances which convert it into an ore, is by 1797.
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Harper, D. (n.d.). Etymology of monometallic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/monometallic