A foam tiara sounds like something you'd buy in Claire's.
"Reversible sequin unicorn foam tiara" or some such.
in the Georgette Heyer novels (which I highly recommend if you like a romantic romp) the ladies are always getting into scrapes and taking their jewellery to be copied in paste so they can pawn the real things for cash.
IIRC Krystle Carrington did this in an early season of Dynasty. The original not the reboot.