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