'Thy mind is a very opal'

'[O]pal . . . [is] traditionally associated with changeability because of the delicate play of colours (as in Shakespeare's Twelfth Night: 'Thy mind is a very opal'). Eliot may also have known of the old story that an opal, wrapped in a bay-leaf, could make the bearer invisible.'

'Helen Small, editor, 1999, of George Eliot's The Lifted Veil, 1859.