Magneton, Nosepass and Charjabug used to have a similiar evolution method, requiring a special magnetic field to evolve into Magnezone, Probopass and Vikavolt, which can be found in places such as Mt Coronet, Chargestone cave or artificially generated in the Kalos power plant, before they too got replaced by regular Thunderstones.
I wondered, if Chargestones could be added as a third evolution rock, that can be found laying around, maybe underground, just cause they look cool
and since they are no longer mandatory, and kinda obsolete, so they arent gonna afect the meta in any meaningfull way, and nintendo technically, sort of, basically confirmed, that the energy of those three rocks is similiar to the respective evolution stones, if they could be given the optional ability to facilitate ALL stone-based evolutions of the respective type.
In the first season of the anime, where we first see moonstones, there was a giant moonstone which was being worshipped by a group of Clefairy (and a weird Scientist guy? Am i remembering that right?) which could be used as a moonstone variant to the evolution rock, and it shouldnt be too hard to imagine a volcanic rock of sorts, that can allow firestone evolutions