quicksilver

Adjective

• 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
    

Noun

• a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures

Related words

• Synonyms: 
  erratic , fickle , mercurial , quicksilver(a) , mercury , hydrargyrum , Hg , atomic number 80