Short of a date monogrammed on your jewelry, the clasp on your antique jewelry is perhaps your most significant indication of the date your jewelry was produced.
A clasp is not just a mechanism. It likely has a patent date and industrial hay day.
The standard for antique Victorian pieces will be the c clasp or “c clutch.” When buying antique jewelry, the c clutch is a great sign. It indicates handcrafted design and genuine age. Since it is generally inferior in mechanics to more modern clasps, its production in the latter half of the 1900s is nearly nonexistent.
Tube and lever catches are also turn-of-the-century designs. The safety catch and spring-ring clasps moved us into a new era in the early 1900s, and by the 1940s, most of the modern clasps we use today were in production.
An excellent illustrated guide to clasps can be found at the url: http://www.morninggloryantiques.com/JewelChatFindings.html.
Before we finish this topic, we should mention that you could have jewelry with a replaced clasp. If you have a piece of jewelry with a more modern clasp that you believed to be an older period piece, then take a good look at the clasp and links surrounding the clasp. If they are slightly different metal or make than the body of the piece, you likely have a replaced clasp.
On most fine antique gold jewelry pieces, you will notice the links are individually soldered (a practice only a few high-end makers such as Tiffany & Co. and Cartier still use today). If you have a fine item like this with a replaced clasp, look at the links that connect the newer clasp to the older links. The replaced links are not usually soldered like the rest of the piece.