Advance Ring
Oracle versions of GB Zelda is for Gameboy Color.
But can be played on GB Advance, too.
If you play on GB Advance, there are an extra shop open.
You will never find this shop open when played on
normal GB color.

Oracle of Seasons: 
Next Door of Birds of Knowledge in Horon Village, you find
a shop that is open when played on Gameboy Advance.
You can buy <Advance Ring "Seasons"> for
100 rupees. 

Oracle of Ages: 
Next Door of Rennu Shooting Gallery in Past, you find
a shop that is open when played on Gameboy Advance.
You can buy <Advance Ring "Ages"> for
100 rupees. 

These rings have no special effect. It's just for fun.