All scallops are Order: Pectinoida in taxonomy. Pearl oysters are Order: Pterioida. Mussels are Order: Mytiloida. (the "oida" suffix means having a shell)
Oyster and mussel shells (and pearls) are largely structured in layers of terraced aragonite and protein. Hence they may be defined as highly nacreous. All mollusks (and some annelids) are considered nacreous, but some to a greater degree than others, particularly Class: Cephalopod and some Class: Nudibranch, which have evolved away from shells, favoring mobility.
Scallops stray from this type of structure. Their shells have nacreous surfaces, but only where soft tissues attach and along margins of new growth. Very soon during each growth cycle, they'll structure their shells (and pearls) with foliated calcite. It's every bit as strong as other structures and quite elegant in different ways. Nacreous surfaces are like tiles on a floor or bricks in a wall, where foliated calcite appears more like rose pedals or fan shapes with a deck of cards.