Have you ever tossed a grade 1 gem bomb onto a pile of rubble/small structure and destroyed it? Some of them have a lost amulet within, but only for one of the modes. (it varies between these). there is at most one amulet/map.
Map 1 has three such structures, though they don't have any lost amulets.
if the above is of no help, you may be thinking of the shrine-burst amulets, obtained by placing a grade 6 or higher pure gem into a shrine.