Old English grindan "to rub together, crush into powder, grate, scrape," forgrindan "destroy by crushing" (class III strong verb; past tense grand, past participle grunden), from Proto-Germanic *grindanan (source also of Dutch grenden), related to ground (v.), from PIE *ghrendh- "to grind" (source also of Latin frendere "to gnash the teeth," Greek khondros "corn, grain," Lithuanian grendu, gręsti "to scrape, scratch"). Meaning "to make smooth or sharp by friction" is from c. 1300. Most other Germanic languages use a verb cognate with Latin molere (compare Dutch malen, Old Norse mala, German mahlen).
"organelle of cells in which biochemical processes occur," 1901, from German, coined 1898 by microbiologist Carl Benda (1857-1933), from Greek mitos "thread," a word of uncertain etymology, + khondrion "little granule," diminutive of khondros "granule, lump of salt" (see grind (v.)).
word-forming element in scientific compounds meaning "cartilage," from Latinized form of Greek khondros "cartilage" (of the breastbone), also "grain, grain of salt, seed, barley-grain," of uncertain origin. This is sometimes said to be from the PIE root meaning "to grind" which is the source of English grind (v.), but there are serious phonological objections and the word might be non-Indo-European [Beekes, "Etymological Dictionary of Greek"]. The body material so called for its gristly nature.
"revolving stone disc used for sharpening, etc." is from c. 1400. Phrase nose to the grindstone in use by 1530s; originally to get control of another and treat him harshly:
This Text holdeth their noses so hard to the grindstone, that it clean disfigureth their Faces. [John Frith, "Mirror to know Thyself," 1532]
The phrase's main modern (reflexive) sense of "working hard" is from 1828.