1670s as the name of a black mineral, oxide of manganese (used from ancient times in glassmaking for removing coloring matter), from French manganèse (16c.), from Italian manganese, alteration or corruption of Medieval Latin magnesia (see magnesia). From 1783 in English as the name of the metallic element.
From the middle of the eighteenth century the combinations of manganese were studied by various chemists, and finally, in 1774, the metal manganese was isolated by Gahn, but for years there was much confusion in regard to its specific name, and it was not until after the beginning of the present century that the name manganese (mangan in German) began to be generally adopted. [Century Dictionary, 1897]
Related: Mangano-, manganic, manganous, manganate.
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.