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
There are several references in this scene to different types of cloth and their different uses. Orsino refers to "the spinsters and the knitters in the sun/And the free maids that weave their thread with bones," alluding to a very pragmatic type of production and a hardwearing, everyday fabric. In Feste's song, he sings of a "shroud of white" cloth whose purpose is to cover a dead body. This is in response to Orsino's comments about the spinsters and knitters.
A few lines later, then, the audience still has in mind these previous references when Feste asks that "the tailor" should "make [Orsino's] doublet of changeable taffeta." He is not actually comparing Orsino to taffeta—rather, he is saying that he should have his tailor make his clothing out of taffeta exactly because it is "changeable." Feste's suggestion is that Orsino should display through his clothing exactly what kind of person he is, inasmuch as the simple fabric woven by the spinsters appears exactly as it is.
Feste also states exactly the way in which he thinks Orsino is "changeable." He says that Orsino's "mind is a very opal." So, his opinion of Orsino is that he cannot be relied upon; just as an opal looks a different color from different angles, Orsino changes his mind depending on the context in which one encounters him. Because he is of "such constancy," Feste feels he should be "put to sea" where his "intent" could truly be "everywhere"—that is, because Orsino has no direction and changes his intent continually, he would be well-suited to a life spent upon the sea, allowing it to carry him wherever it would.
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.