Symbolism, Imagery, Allegory
There's a lot of bling going around in Twelfth Night. Come to think of it, Olivia's the one who seems to be always handing it out. First, she gives a ring to "Cesario" (which is a secret signal that Olivia's has a crush on "him"). Then, she gives "Cesario" a miniature (a piece of jewelry with Olivia's tiny portrait on it). Duke Orsino also gives "Cesario" a jewel to pass along to Olivia. In these cases, the unwanted trinkets are emblematic of the giver's love, which is ultimately rejected.
Finally, though, when Olivia gives a pearl to Sebastian, Sebastian is pleased as punch, a pretty clear signal that he loves Olivia back. Too bad Olivia thinks she's given "Cesario" the pearl. By this time, love trinkets seem to have lost all meaning because everybody's just giving the stuff away like nothing. Rather than being a cherished symbol, jewels and hearts (the metaphorical kind, not the ones beating in our chests) become just another thing to be given carelessly and foolishly.
Malvolio's fantasy of fondling "some rich jewel" is a bit different. For Malvolio, a servant who desperately wants to marry up so he can boss people around, jewels signify power, not love. Consider this passage where Malvolio daydreams about what it would be like to have more authority than Sir Toby if he married the Countess Olivia:
Seven of my people, with an obedient start,
make out for him. I frown the while, and perchance
wind up watch, or play with my—some
rich jewel. Toby approaches; courtesies there to me—(2.5.7)
Here, Malvolio is about to say that he would act disinterested in Toby by "play[ing]" with his steward's chain (a chain worn by stewards to signify their status as head servant) while Toby bows before him. (We know that Malvolio walks around fondling his steward's chain because Sir Toby tells him to "go rub" it with "crumbs" in Act 2, Scene 3. You can read more about this quote under the theme of "Society and Class.") But, in his fantasy, Malvolio quickly corrects himself – if he was married to Olivia, he wouldn't be wearing a menial steward's chain. So, Malvolio replaces the chain with "some rich jewel," a much more appropriate symbol of his status in this fantasy. Of course, the text also implies that Malvolio's constant "rub[bing]" and "play[ing]" with his chain and the make-believe jewel suggests that Malvolio is more excited about power more than anything else.