Dee, pyrite is sulfur and iron, almost a recipe for oxidation.
Exposure to air will cause it to soon begin changing color.
Additionally, it is likely to be fragile. Used in a ring, the ammonite
would probably break soon, possibly while trying to set it (this from
personal experience setting one for a pendant). The better you seal
the entire ammonite the longer the finish will last.
As gratuitous overkill, the pyrite ammonites are coming from
Mikhailov, Russia. Their age is Middle Jurassic, Upper Callovian and
the species noted from there by Jim Wyatt, owner of fossilnet.com, are
Binatisphinictes rjazanensis, Cosmoceras compressum, C. proniae and
Now aren’t you sorry you asked?