Platinum was identified as an element in 1741, but the word for the metallic substance didn't come into English until 1812; they didn't even have a word for it in Old English. This wasn't borrowed the usual route, that being directly from French or Latin; actually, it was taken from the Spanish diminutive platina, meaning "little silver". This is because platinum was considered an impurity when found in silver, and it was sort of meant pejoratively. Platina is from plata, meaning "silver" in general, and that, through either Old French or Old Provençal, is related to our word plate through Latin plata, meaning either "plate" or "piece of metal" (and thus our etymologies join). This likely comes from Ancient Greek platys, meaning "wide", "flat", or "broad" (also the prefix in plateau, platonic, platform and platypus), which derives from a Proto-Indo-European reconstruction sounding like plat and meaning "to spread". Through a lot of changing definitions and relatives, the etymology of platinum has enjoyed a nice spread.
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