In Shakespeare's Hamlet, in the last act, Claudius and Laertes have made arrangements for a an afternoon of "sword play" to be carried out as a form of entertainment. It is to be a civilized meeting—a friendly activity. However, Laertes is looking to avenge his father's death, and Claudius has encouraged him to take Hamlet's life.
At the start of the play, Claudius seemed relatively benign (except for the fact that we know he is a murderer), but as the plot starts to accelerate, he begins to manipulate those around him to bring Hamlet to his death. The King orders Rosencrantz and Guildenstern to escort Hamlet to England; they carry a request from Claudius to England's king to execute Hamlet. Hamlet changes the message, so that his escorts are executed instead. To Claudius' surprise and annoyance, Hamlet returns home. Claudius makes his plans with Laertes so that Hamlet will be cut with a poisoned sword, thus bringing about Hamlet's end.
However, to be certain that Hamlet dies, Claudius also has wine at his throne. At a break in the "game," Claudius drops a pearl in a cup of wine, saying to Hamlet:
Stay, give me drink. Hamlet, this pearl is thine;
Here's to thy health. (V.ii.283-284)
The King puts a pearl in the cup ostensibly to show his faith in Hamlet's ability to win, and to reward his stepson—at least this is how he plays the audience that is gathered there to watch. However, the pearl is poisoned. Claudius is desperate to see Hamlet dead, so he has arranged a backup plan in case Laertes is unable to poison Hamlet. Hamlet refuses the drink, wanting to stay clear-headed as he and Laertes "fight." However, Gertrude reaches for the cup to toast to her son. Claudius tells her not to drink, but he is not forceful enough, and Gertrude is poisoned instead.
When Claudius says, "This pearl is thine," I always assume that literally he is saying so as if to bestow a gift on Hamlet. However, I believe he is thinking, "This pearl is meant for you, to see you dead." Because the pearl itself is seemingly innocuous, it would appear to everyone to simply be a gift. That it is not, supports the theme of appearance vs reality (or "reality vs illusion").
We have seen this theme throughout the play. When Hamlet pretends to be insane when he it suits him, he appears mad, but the reality is that he is trying to garner information in order to see if the Ghost is honest, and if so, to avenge Old Hamlet's death.
When Ophelia speaks to Hamlet, it might seem to the average onlooker that they are merely exchanging pleasantries, however everything Hamlet says will be relayed back to Polonius and the King. Rosencrantz and Guildenstern pretend to be Hamlet's concerned friends, but everything they do is for the King, with no concern for anyone but themselves. In the last scene of the play, Claudius seems supportive of his "son." Laertes seems willing to put away the bad blood between them, but it is an act. All of these situations are examples of the theme of appearance vs reality.
The pearl seems like a reward, but it is actually a deadly "weapon," killing first Gertrude and then Claudius.