quicksilver

nouns

a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures
Hg, atomic number 80, hydrargyrum, mercury, quicksilver

adjectives

liable to sudden unpredictable change
"erratic behavior"; "fickle weather"; "mercurial twists of temperament"; "a quicksilver character, cool and willful at one moment, utterly fragile the next"
erratic, fickle, mercurial, quicksilver