As a general rule, signatures without a copyright symbol indicate the piece was manufactured prior to 1955. Jewelry companies would often use up their supply of pre-stamped clasps and findings after switching to a new signature, so some pieces made soon after 1955 might not have copyright symbols.
A detailed study of vintage jewelry marks could fill multiple books, but to give some guidance we can use the Trifari company as an example.
Whether you plan to keep the jewelry as a family heirloom or would like to resell it, a knowledge of vintage jewelry marks will help you to identify and date it properly.
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Antique silver hallmarks have been used to control the quality of goods made of silver since the 14th century and the organisation that regulates the craft, Goldsmiths Hall, gave the world the term hallmark.
They have an extensive library of jewelry marks located here: Researching Costume Jewelry.
Just scroll down a little and click the letter of the alphabet for the company marks you wish to view.
Vintage costume jewelry usually doesn’t have purity marks, but will often have a maker’s mark (commonly called the signature), and can sometimes have a retailer mark and/or patent or inventory marks.