Nice brooch, I would say 1940s - Many less costly stones were used with open back settings, these are all unfoiled stones & came in crystal, glass & basic rhinestones so an open back does not always mean a more expensive stone was used like diamonds. A quick test for diamonds if you place the piece close to your mouth and breath heavy on the stone like polishing reading glasses a true diamond will barely fog and clear quickly other clear stone totally fog and clear slowly, This is just an indicator to have a jeweler inspect the stone if fog test indicates it may be a diamond. Your piece from the look of the metal and stones I would say rhinestone set in gold tone metal. Here is a piece of mine done with open backs in crystal.
