In "Twelfth Night," why does Feste compare Duke Orsino to an opal and taffeta, then say that he wishes such men were put to sea? Act 2, scene 4
Feste is commenting on the inconstancy or fickleness of the Duke and men in general. He says,
"Now, the melancholy god protect thee, and the
tailor make thy doublet of changeable taffeta, for thy
mind is a very opal. I would have men of such constancy
put to sea, that their business might be everything
and their intent everywhere, for that's it that
always makes a good voyage of nothing. Farewell." (2.4)
According to David Bevington's footnotes in The Necessary Shakespeare, taffeta is "a silk so woven of various-colored threads that its color shifts with changing perspectives," and an opal is "an iridescent precious stone that changes color when seen from various angles or in different lights." And according to the eNotes link below,
"Interestingly, in spite of the Duke’s praise for this song, the Clown insults Orsino in a manner similar to the way he insulted Olivia in Act I. The Clown suggests that he lacks consistency and direction, though the logical form of his expression is not so apparent as in his insult to Olivia."
Feste is comparing the Duke to these two items - taffeta and an opal - that change in appearance depending on the light of day or the way someone is looking at them. He is calling Orsino fickle.
Finally, he wishes that men like this could be put to sea, where nothing is constant - the sea changes continually and never looks exactly the same from moment to moment.