Many manufacturers stamped (signed) their jewelry. These signatures, where present, are found on the reverse of the piece, either on a dip back or on the body itself. They can commonly be quite small and difficult to see and usually require a small hand lens, or "loupe", to read. Many markings on the back of vintage costume jewelry are, however, not signatures identifying the manufacturer but could be part numbers, patent numbers, or even casting faults, so look carefully.
Makers' marks can also tell us a great deal about the age of a piece. A few manufacturers (such as Dior and Hollycraft) actually stamp the year of manufacture with the regular signature, while others have, over the course of the company's life, changed the design of the signature, thus providing us with a useful method of estimating the age of a piece. This is not an exact science, because a maker could use the same design of signature for over 20 years, but with that knowledge and other information such as design, colour, method of manufacture, and material, one can narrow down the estimated age to within five years.
There may also be other useful information stamped on the reverse of the piece. Costume jewelry made from
silver generally bears either a hallmark (if made in the UK) or other marks depending on country of origin. These range from the grade of silver written in numbers (for example 925 for sterling silver or 800 for lower-grade silver), assay office marks of individual countries (far too numerous to list here), or "STERLING SILVER" for sterling silver pieces made in the United States. Grade and thickness of gold plating is found on some pieces.
Finally, it is important to note that just because a signature cannot be found on a piece of jewelry it doesn't mean that it is not of any value. Some of the finest and most valuable costume jewelry is "unsigned". After all, somebody must have made it and very many makers simply did not stamp their pieces. Sometimes the process of plating a piece (mostly done after the signature had been stamped) can fill in the signature, all but obliterating it, and of course signatures can wear away over time. This is where real skill and judgment come into play, and after a fair amount of practice one may be able to identify manufacturer, designer, and age with just a brief glance. Some of the more common makers marks are listed opposite with approximate dates of usage.