Two histories under one spelling
English uses rock for a cliff, a handheld stone, a diamond, the motion of a cradle, and a style of loud popular music. Those meanings do not form one family. The language keeps two unrelated words that converged on the same short form, plus a third, rarer homonym for a spinning distaff. The mineral noun is a medieval borrowing. The verb of swaying is native Germanic. The music sense grew out of the verb.
A Romance noun for a mass of stone
When people look up the origin of the word rock and expect geology, they are chasing the noun. Middle English wrote it rokke or roche and used it for stone as a substance and for a large crag, outcrop, or rocky height. The form sits at the meeting of two streams. Old English already had rocc, but the evidence is thin: it is best known inside the compound stanrocc, a gloss for an obelisk or a high stone peak. After the Norman Conquest the word was reinforced, and on some accounts reborrowed, from Anglo-Norman rokke and northern Old French roke, beside the more widespread Old French roche.
Those French types match Medieval Latin rocca, attested from the late eighth century, and a reconstructed Vulgar Latin *rocca. What lies behind that Latin word is not settled. A Celtic source has been proposed, with Breton roc'h often named as a comparison, but that account is not generally accepted. An older suggestion derived it from Latin rupes through a hypothetical *rupica; that too remains a guess. Standard dictionaries now treat the deeper source as unknown.
Once the noun was in English, it did not replace native stone. For a long stretch it named large masses: cliffs, crags, reefs, and only sometimes a single boulder. Calling any pebble a rock is later. American colloquial English had that wider sense by the 1790s, and nineteenth-century usage books still treated it as a fault. In the stricter view, a rock was something too heavy for an ordinary person to lift.
From the sixteenth century the same noun could stand for safety or for wreck. Biblical English used it as a figure for a sure foundation, especially of Christ. At the same time, ships that struck rocks gave it a sense of peril. Both images were already in play in the 1520s. The American phrase between a rock and a hard place, recorded in the Southwest around 1914, belongs to the danger side: two hard options and no clean exit.
Slang kept scaling the noun down. Precious stone, especially a diamond, is American slang from the early twentieth century. A crystallized lump of cocaine is later, from West Coast usage in the 1970s. Those senses needed no new etymology; they are the old mass-of-stone word made smaller or more figurative.
Romance cousins remain easy to see: French roc and roche, Spanish and Catalan roca, Italian roccia and the fortress sense of rocca, Dutch rots. They all point back to the same poorly explained medieval Latin type. English also kept a related borrowing roche in older and technical use, but everyday speech settled on rock.
A Germanic verb for a cradle
The verb has nothing to do with cliffs. Late Old English roccian meant to move a child gently to and fro in a cradle. Middle English rokken kept that sense. The verb is inherited from West Germanic, reconstructed as *rukkōn, itself from Proto-Germanic *rukkōną. Related forms include Old Norse rykkja, Swedish rycka, Middle Dutch rucken, Old High German rucchan, and modern German rücken, which tends toward a jerk or a shift rather than a lullaby.
The first English associations were rest, not violence. Transitive rocking of a cradle is the oldest use. An intransitive sense, of something swaying unstably on its own, appears by the late fourteenth century. Ships rocking in waves are recorded from the early sixteenth century. The warning not to rock the boat, meaning not to stir up trouble, is early twentieth-century American. Sitting in a rocking chair, and the verb used of that motion, belong to the late eighteenth century.
A separate, minor verb from the stone noun, meaning to pelt with stones, exists in later English. It is a conversion of the noun, not a continuation of roccian. The two verbs share spelling and little else.
From slumber to a beat
In the twentieth century the cradle verb took on a public life. Blues slang in the 1920s used it for moving with a musical rhythm, often with a sexual charge. By the late 1940s it meant dancing to popular music with a strong beat. Radio and the record trade, including the disc jockey Alan Freed around 1954, spread rock and roll as a name for a style that had often been sold as rhythm and blues. The shortened noun rock for that music is in place by the late 1950s. Later labels such as folk rock, hard rock, and punk rock are compounds on that shortened noun, not on the geological one.
Later slang from the same verb — a performance that rocks a crowd, clothes someone is said to rock — still tracks motion and impact, not mineral matter.
A third homonym, quickly
English also inherited a noun rock meaning distaff, the staff that holds flax or wool for spinning. That word came through Middle English from Middle Dutch rocke and is akin to Old High German rocko. It is now rare outside historical discussion, but it is a genuine third item, not a sense of either the stone or the cradle word.
One dating worth keeping
If a single fact is worth carrying away, it is the Old English compound stanrocc. The mineral noun was already in the language before French became the obvious donor, yet it was scarce, perhaps only in that compound, until Anglo-Norman and Old French gave it new bulk. The verb needed no borrowing. English has been rocking cradles for as long as it has been English; it learned to call a crag a rock with help from the Continent.