quicksilver

(noun) a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures. Synonyms: mercury, hydrargyrum, Hg, atomic number 80.

(adj) liable to sudden unpredictable change. Synonyms: erratic, fickle, mercurial, quicksilver(prenominal). Examples: "Erratic behavior." "Fickle weather." "Mercurial twists of temperament." "A quicksilver character, cool and willful at one moment, utterly fragile the next."