rock-candy n.

[stone, mass of mineral matter], Middle English rokke, roche "stone as a substance; large rocky formation, rocky height or outcrop, crag," from Old English rocc (as in stanrocc "stone rock or obelisk") and directly from Old North French roque, variant of Old French roche, which is cognate with Medieval Latin rocca (8c.), from Vulgar Latin *rocca, a word of uncertain origin. According to Klein and Century Dictionary, sometimes said to be from Celtic (compare Breton roch). Diez suggests Vulgar Latin *rupica, from Latin rupes "rocks."

In Middle English it seems to have been used principally for large rock formations but occasionally of individual boulders. The extended sense of "a stone of any size" is by 1793, American English colloquial, and long was considered incorrect.

It is an error to use rock for a stone so small that a man can handle it : only a fabulous person or a demi-god can lift a rock. [Century Dictionary]

The meaning "precious stone," especially a diamond, is by 1908, U.S. slang; the sense of "crystallized cocaine" is attested from 1973 in West Coast slang. Also used attributively in names of animals that frequent rocky habitats, as in rockfish, rock badger, rock lobster (the last attested by 1843).

Rock is used figuratively for "a sure foundation, something which gives one protection and security" (especially with reference to Christ), from the 1520s (Tyndale); but it also has been used since the 1520s as "cause or source of peril or destruction," an image from shipwrecks.

Between a rock and a hard place "beset by difficulties with no good alternatives" is attested by 1914 in U.S. Southwest:

to be between a rock and a hard place, vb. ph. To be bankrupt. Common in Arizona in recent panics; sporadic in California. [Dialect Notes, vol. v, part iv, 1921]

As an example of fine distinctions, a party of men were discussing the present situation of the German army, this week. One remarked that the Germans were between the devil and the deep sea; while another corrected him by saying that the Germans were between the upper and nether mill stone. The third man whose name is Pilgreen, and who works in the treasurer's office, simply remarked that the Germans were between a rock and a hard place. [local item in the Pouteau (Oklahoma) Weekly Sun,  Oct. 1, 1914]

The rock-scissors-paper game is attested by that name by 1976 (as paper stone and scissors by 1941). Sources agree it is based on Japanese Jan Ken Po or Jan Ken Pon (or Janken for short); the Japanese game is described in English publications by 1879.

late 13c., "crystallized sugar," from Old French çucre candi "sugar candy," ultimately from Arabic qandi, from Persian qand "cane sugar," probably from Sanskrit khanda "piece (of sugar)," perhaps from Dravidian (compare Tamil kantu "candy," kattu "to harden, condense").

The sense gradually broadened (especially in U.S.) to mean by late 19c. "any confection having sugar as its basis." In Britain these are sweets, and candy tends to be restricted to sweets made only from boiled sugar and striped in bright colors. A candy-pull (1865) was a gathering of young people for making (by pulling into the right consistency) and eating molasses candy.