This is the Adventures of Kelvin of Rud series by Piers Anthony and Robert E. Margroff.
The books are:
Dragon's Gold
Serpent's Silver
Chimaera's Copper
Orc's Opal
Mouvar's Magic
So the book you remembered had "Opal" in the title is Orc's Opal. The books do indeed dwell unnecessarily on the shapes of people's ears.