I don't have the books so I can't check the plot for details like ear shapes, but a possibility would be 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

so they are kind of jewellery related and the fourth book is indeed someones Opal

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.