It is hard to specifically date a piece of jewelry by the clasp alone, unless it is marked in some with with a designer hallmark. The reason for this is that most of the clasps used on vintage jewelry are also used on modern pieces. Condition of the metal and other design techniques in the piece should be used in conjunction with a clasp to try and narrow down the date.
Vintage jewelry necklaces were made with many styles of clasps. Most notable were spring ring, hook clasp, box clasp, fold over clasp, and fish hook clasp. Others used, but not as commonly, were barrel clasp and toggle clasps. Lobster claw clasps were a later invention, so they are not found on vintage pieces, unless the clasp has been changed at some point.
Hook clasps were often used on multi strand necklaces from the 1950s and 1960s. They were made of a metal hook which either "grabs" a bead in a dangling strand, or hooks in to a metal chain. The latter is still commonly used, but the former usually dates a piece as a vintage one.
A fish hook clasp is made with a small hook which is inserted into a metal finding, often a pointed oval shape. Good vintage fish hook clasps were often sterling silver. Modern metal ones are lighter in weight and made from goldtone and silvertone metal.
Fold over slaps were used for many vintage mass produced pieces. The underneath part of the clasp was a great place to put the designer hallmark. They were commonly used on necklaces and bracelets. On a bracelet, a fold over clasp was often used in conjunction with a safety chain to help prevent loss of the bracelet if the clasp open while the piece was being worn. This type of clasp was popular in the 1940s and is still used today.
1. Collecting Vintage Jewelry - Where do I start?
2. Collecting Vintage Jewelry - What is the condition?
4. Collecting Vintage Jewelry - The Creative Use of Stones.