Could be. Which is probably a better answer than you expected, given that that costume generally is taken to mean “fake” which often equates to worthless. Take the time to go through Grandma’s jewelry and you might find some surprising treasures.
While some costume jewelry isn’t worth much, other stuff—especially that kind made by actual designers—can be worth more than undesirable fine jewelry that ends up being sold for scrap.
Jewelry from before the 1930s, no matter the kind, can be called antique, while jewelry after that is vintage costume. That’s not a hard and fast rule as much as a rule of thumb. Not all vintage jewelry is worth money, but you can get a good idea of what is by checking the pieces for maker’s marks, and then comparing similar pieces for sale on auction sites, or online jewelry guides.
Signed pieces by designers like Miriam Haskell, William Hobe, or Hattie Carnegie are going to bring the most money. And pieces with all the rhinestones intact and a nice finish still on the metal will fetch higher prices as well. Art Deco and Victoria costume jewelry in good condition is always in demand.
Do your homework; know what pieces are worth, and you may find you become just as interested in seeking out fun costume pieces at Northern Virginia pawn shops like Loudoun Jewelry on Maple Avenue as you on in selling—maybe more!