Is there really an exchange between Macbeth and Banquo in act II scene1?
Banquo gives Macbeth a diamond, which Duncan has asked be given to Lady Macbeth for her boundless hospitality as hostess to the King and entourage:
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.
Banquo gives Macbeth a diamond
Since Duncan is "a-bed" Banquo is the highest ranking thane on the watch; therefore, he must carry out Duncan's royal order here.
The exchange is, of course, ironic since the audience has just heard Lady Macbeth and Macbeth plot to murder Duncan in his sleep.
I always felt that an exchange was unlikely. When Banquo says "This diamond he greets your wife withal..." he could just have been referring to a gift given to Lady Macbeth by Duncan when he first greets her upon his arrival at Macbeth's castle. It's sort of like saying, "This great mood he was in..." It never made sense to me that Banquo would be in the possession of the diamond, especially as he was about to go to sleep.