semiprecious (adj.)

before vowels often sem-, a word-forming element of Latin origin meaning "half," also loosely, "part, partly; partial, almost; imperfect; twice," from Latin semi- "half," from PIE *semi- "half" (source also of Sanskrit sami "half," Greek hēmi- "half," Old English sam-, Gothic sami- "half").

The Old English cognate, sam-, was used in such compounds as samhal "in poor health, weakly," literally "half-whole;" samsoden "half-cooked" ('half-sodden'), figuratively "stupid" (compare half-baked); samcucu "half-dead," etymologically "half-alive" (see quick (adj.)); and the lingering survivor, sandblind "dim-sighted" (q.v.).

The Latin element was common in formations from Late Latin, as in semi-gravis "half-drunk," semi-hora "half hour," semi-mortuus "half-dead," semi-nudus "half-naked," semi-vir "half-man, hermaphrodite."

The Latin-derived form in English has been active in forming native words since 15c. Semi-bousi "half-drunk" ('semi-boozy'), now obsolete, was among the earliest (c. 1400). As a noun, semi has variously been short for semi-detached house (by 1912), semi-trailer (by 1942), semi-final (by 1942).

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.