What was Big Willy Shakes going for?
This casket's a wise one. Essentially, the inscription is saying that what's golden outside isn't always golden inside, and the chooser would've known that had he been as wise as he is bold. Ouch. That zinger's gotta hurt. But to be fair, he is a prince and we know that princes like shiny, fancy stuff like gold.
Since he's only in this scene of the play, the Prince has come to act as a sort of symbol in the play, rather than a real, fleshed out character. He stands for anyone who is more interested in a person's wealth and material possessions than what's on the inside. (In this case, the caskets make this all rather literal.)
We should tell you that there's a lot more going on here than just picking the right color casket. The three caskets (gold, silver, and lead) are major symbols in the play, too. The big tipoff is the fact that each of them is inscribed with a message on the outside and also contains a note on the inside. Our Big Willy was all about the power of words.
We've already said the blinged-out gold chest tells us appearances are often deceiving, and human desire (for wealth, sex, or what have you) can be dangerous. But what about the other two?
Well, the inscription on the outside of the silver chest reads, "Who chooseth me shall get as much as he deserves." The inside contains a picture of an "idiot," with a nasty little note: "So be gone: you are sped. Still more fool I shall appear by the time I linger here with one fool's head I came to woo, but I go away with two" (2.9.3). In other words, whoever chooses the silver casket is a fool who'll get what he deserves (a picture of another fool). The lesson here just might be that if you think you're someone who deserves a great deal, you'll wind up with, well, not much.
Finally, the lead chest, which is made of a very humble metal, seems to symbolize inner beauty and modesty (the exact opposite of the shiny gold casket). It contains a picture of Portia. The inscription is also significant: "Who chooseth me must give and hazard all he hath" (2.7.5). Gee, this sounds like a pretty good description of marriage: a big risk that requires a lot of sacrifice.
While "all that glitters is not gold" is the most famous phrase in this scene, it's really just one part of the larger symbolism of the three caskets. Just like no man is an island, one casket can't stand alone.
Shakespeare might have come up with this phrase, but the idea was well known in England before he put words to it. In the 12th century, it went a little something like this: "Do not hold everything gold that shines like gold." And Geoffrey Chaucer wrote, "Hit is not al gold, that glareth." But we think Shakespeare's version has a nice ring to it, don't you?
In fact, Shakespeare's original version had "glisters" instead of "glitters." This is just a fancy way of saying the exact same word.