late 14c., in alchemy, "one of the two main ingredients of the philosopher's stone," from Medieval Latin magnesia, from Greek (he) Magnesia (lithos) "the lodestone," literally "(the) Magnesian (stone)," a mineral said to have been brought from Magnesia, the region in Thessaly, which is said to be named for the native people name Magnetes, which is of unknown origin.
The ancient word, in this sense, has evolved into magnet. But in ancient times the same word, magnes, was used of lodestone as well as of a mineral commonly used in bleaching glass (modern pyrolusite, or manganese dioxide).
In the Middle Ages there was some attempt to distinguish lodestone as magnes (masc.) and pyrolusite as magnesia (fem.). Meanwhile, in 18c., a white powder (magnesium carbonate) used as a cosmetic and toothpaste was sold in Rome as magnesia alba ("white magnesia").
It was from this last, in 1808, that Davy isolated magnesium. He wanted to call it magnium, to stay as far as possible from the confused word magnesia, but the name was adopted in the form magnesium. Meanwhile from 16c. the other name of pyrolusite had been corrupted to manganese, and when, in 1774, a new element was isolated from it, it came to be called manganese.
Magnesia in its main modern sense of "magnesium oxide" (1755) is perhaps an independent formation from Modern Latin magnes carneus "flesh-magnet" (c. 1550), so called because it adheres strongly to the lips.
1880, proprietary name for white suspension of magnesium hydroxide in water, taken as an antacid, invented by U.S. chemist Charles Henry Phillips. Herbal or culinary preparations more or less resembling milk had been similarly named (for example milk of almond) since late 14c.
rock made of carbonate of calcium and magnesium, 1794, named for French geologist Déodat De Gratet De Dolomieu (1750-1801) who described the rock in his study of the Alps (1791). Dolomites for "mountains of Southern Tyrol" is from 1870. Related: Dolomitic.
c. 1400, name of a plant reputed to contain antivenom, often identified as dragonwort, from Old French serpentin name of a precious stone, a noun use of adjective meaning "of a snake, snake-like; sly, deceptive," from Late Latin serpentius "of a serpent," from Latin serpentem (nominative serpens) "snake" (see serpent). Also in some instances from Medieval Latin serpentina. From mid-15c. as the name of a kind of cannon used 15c.-16c.
As the name of a greenish metamorphic rock consisting mainly of hydrous magnesium silicate, it is attested in English is by c. 1600, perhaps based on Agricola's Lapis Serpentinus (16c.). Earlier references in English are to a precious or semiprecious stone thought to have magical powers (early 15c.) but these were perhaps from the translucent ("noble") form of the mineral. The name is perhaps in reference to the rock's green color, though some sources write of "markings resembling those of serpent's skin" or "similarity of the texture of the rock to that of the skin of a snake."
An ancient name for the rock is said to be hydrinus, perhaps suggesting connection to the sea-serpent hydra. It also has been identified with classical ophitēs, a ornamental building-stone mentioned by several writers, related to ophis "serpent, a snake" (see ophio-), but this is uncertain: Pliny said it has marking like a snake, but he included it among the marbles.