mid-13c., "valuable, of great worth or price, costly," from Old French precios "precious, costly, honorable, of great worth" (11c., Modern French précieux), from Latin pretiosus "costly, valuable," from pretium "value, worth, price" (see price (n.)).
The meaning "over-refined, fastidious" in English is by late 14c. From 16c. through 18c. it also had a secondary ironic (inverted) sense of "worthless." Precious metals (1776) "gold and silver (and sometimes platinum)" are those that are rare and costly enough to be used as a standard of value and abundant enough to be used for coinage. Related: Preciously; preciousness.