They are imitation pearls. You can see where the coating has peeled off some of the pearls near the clasp.
Ladies of that generation frequently wore imitation pearls because genuine cultured akoya pearls were expensive. Good quality akoyas are still costly, but nowadays people have the option of buying nearly-round /round freshwater cultured pearls; back then this was not an option.
These may have sentimental value for you, and if so, you should feel free to wear them!
Did you know that Jackie Kennedy's famous 3-strand pearl necklace was an imitation, made by Kenneth Jay Lane? Same goes for Barbara Bush's pearl necklaces.