As far as his fantasy novels go, he has a certain fixation on little blue gems of immense and unbelievable power that cannot be handled by mortal man.

In the Belgariad and the Mallorean, it was the Orb of Aldur. Only the Gods or Belgarion could handle the little pebble in question.

In the Elenium and the Tamuli, it was the Bhelliom. This little blue flower made of Sapphire and could only be handled by either Sparhawk or the Gods.

In both series, these little blue objects are parts of forces beyond comprehension, both with polar opposites. They have powers beyond reason. They are plot devices.
I do love all the books that David and Leigh wrote. I'm just jaded about this little blue gem complex.
Edit: someone pointed out to me that the Bhelliom is made of Sapphire. Which is why it's called the Sapphire Rose. Duh me.