Ion stones
Stones that orbit a head and do useful things, and the party has never owned one
Small magic stones that hang in the air about a person’s head, and there is a limit to how many of them one head will carry. What they do varies widely: sharpening an attribute, swallowing a spell aimed at the wearer, letting them go without breathing, or simply making them better at some ordinary skill. There are a great many kinds.
When the electrum circlet was finally identified, part of what it turned out to do was raise that limit by two. Circlets of the sort are not rare, and not common either; they are the kind of thing a good craftsman buys: light to work by after dark, and a couple of extra stones to carry whatever their trade rewards.
The party has never held one. The two slots the circlet opens have stood empty since the day they learned the circlet opened them.