When a piece of costume jewelry bears the maker's mark it is far easier to determine when it was manufactured. Many companies changed their signatures from decade to decade. For example, a Hattie Carnegie brooch marked with a capital HC inside a small diamond shaped symbol was made significantly earlier than one bearing her full name engraved in script lettering. Additionally, most jewelry companies didn't add the copyright mark; a small circle with a "c" inside it, until 1955. But here is a tip for determining the age of a piece of costume jewelry that most people do not know. Some rhinestones have a rainbow coating on the surface which reflects colors of blue, pink, lavendar, green or gold. This finish is called "aurora borealis" and adds another dimension to glass rhinestones.  The finish went into production around 1953 and has been used ever since. So, if you find a piece of rhinestone costume jewelry and are told it dates to 1920 but it has aurora borealis rhinestones, you will know the date is inaccurate by about 33 years! - Burcaggio House