Good eye.
I did some testing and i can safely say that it is just only have one base(plain rock) meaning rock+anything will get you the earring.

It is just confusing since the Azure Earring has 2 different base stat giving at the website. Each of them works but the earring just has one base only.
http://wlodb.com/items/5378
http://wlodb.com/items/5388
The comment itself even says that it has one base only as well.