magnet: [15] Greek Mágnēs líthos meant ‘stone from Magnesia’ – Magnesia being a region of Thessaly, Greece where much metal was obtained. It had two specific applications: to ore with magnetic properties, and to stone with a metallic sheen. And it was the first of these that has come down to English via Latin magnēta as magnet. English magnesia [14] comes from the same source, but it is not clear how it came to be applied (in the 18th century) to ‘magnesium oxide’, for it originally denoted, in the rather vague terminology of the alchemists, a ‘constituent of the philosopher’s stone’.
In the 17th century it was used for ‘manganese’ (and manganese [17] itself comes via French from Italian manganese, an alteration of medieval Latin magnēsia). And when the term magnesium [19] was introduced (at the suggestion of the chemist Sir Humphry Davy), it too at first denoted ‘manganese’. => magnesium, manganese
