I know many people around here look at it differently. Some shops consider costume jewelry to be gold or silver pieces - a few antique shops. Other places consider it to be just about anything.
I had some nice pieces of what was considered to be costume jewelry around here at an auction. I had what they call High End costume jewelry in some auctions from Monet, Napier, Sarah Coventry, Coro, Trifari and I can't remember who else, but ironically the person who sold it (where I took it) mentioned in the description nothing about it, but yet they mentioned the same designers and being high end on their own stuff they were selling in the auction and of course got good $ out of it. Mine they acted like it was junk, yet it was nice and the same exact designers!