In Act 2, Scene 1, Banquo encounters Macbeth late at night and begins a significant conversation:
What, sir, not yet at rest? The king's a-bed:
He hath been in unusual pleasure, and
Sent forth great largess to your offices.
This diamond he greets your wife withal,
By the name of most kind hostess; and shut up
In measureless content.
Macbeth accepts the diamond which Duncan has entrusted to Banquo and replies:
Our will became the servant to defect;
Which else should free have wrought.
Shakespeare created this dialogue for many purposes. In the first place, it tells the audience that the king has sent forth "great largess" to Macbeth's "offices," i.e., his soldiers and household staff. This will explain why the Porter was so drunk that he couldn't answer the gate when Macduff was knocking. The prolonged knocking at the gate will explain why Macbeth has to come down in person in his nightgown to see what is going on. This will force Macbeth to be present when Macduff discovers Duncan's body and wakes the whole castle. Shakespeare wanted Macbeth to be present when all this happens because it makes for much better drama. Also, it is the only time that the protagonist and antagonist will be together before their death duel in the last act. They have to be shown together to demonstrate that they even know each other.
In addition to explaining why everyone on Macbeth's staff got drunk, it attempts to explain a difficult plot problem, which is: "Why did Macduff have to sleep outside the castle, and why didn't Duncan appoint someone to wake him who would be sleeping inside the castle, someone like Banquo, for instance?" Macbeth tells Banquo they were "unprepared" to accommodate all the important people who arrived with King Duncan. Poor Macduff had to spend the night in some wretched hovel during the worst storm in ages and then stand outside in the cold and rain knocking on the castle gate without getting any response. Shakespeare wanted Macduff to be the man who discovers Duncan's body, and this was the best he could think of. The monologue of the drunken Porter will explain that everybody had been drinking up the liquid part of the King's "largess" and they were still sleeping it off. The Porter gets the audience laughing, so they ask themselves a lot of questions about the plot, such as: "Why does the King travel around with so much liquor?"
The diamond that Banquo presents to Macbeth is only intended to symbolize the extent of Duncan's generosity and "largess." It brings Banquo and Macbeth close together on the stage, where they carry on a conversation fraught with significance. Duncan brings up the subject of their meeting with the Weird Sisters, and Macbeth tries to sound Banquo out about joining him in a coup.
I think not of them:
Yet, when we can entreat an hour to serve,
We would spend it in some words upon that business,
If you would grant the time.
So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counsell'd.
Thanks, sir: the like to you!
Macbeth has suggested that he would like to discuss the Weird Sisters prophecies at some future convenient time, but the discussion is actually going on beneath the surface. Banquo has said, "No!" and Macbeth will have to act alone.