I had the pleasure of giving a presentation at the Minnesota Historical Society, back in 2007. The presentation was sweetheart jewelry of World War II ; about the souvenirs and sentimental items sent back to loved ones during the war. It was especially popular during World War II. During the war many people here at home would wear jewelry reflecting the branch of service a loved one was serving as well as patriotic themed jewelry.
I brought in some examples of sweetheart jewelry, including some pieces my Grandfather made and sent home to my Grandmother. The above image are the bracelets he made my Grandmother from sterling silverware from his ship. Sweetheart jewelry was mass produced or handcrafted by the servicemen.
The war affected the types and availability of materials used for production with critical metals restricted in 1942. It was still legal to use sterling, if companies could find it, or had enough supplies stored. As a result, you will see jewelry made with other materials, such as plastic, wood, mother of pearl, etc. (and sometimes trench art is made from downed aircraft or artillery shell parts) It was harder to find sparkling jewelry as the best glass came from Czechoslovakia (although some companies had some on hand).
Pins are the most commonly found type of sweetheart jewelry. Sweetheart expansion bracelets were very popular during that time. Lockets were another popular treasured item. Harder to find are the earrings that matched the other pieces.
One very common pin is the service pin. These are the pins in patriotic colors with one, two, three or four blue stars. The stars represent number of those in service. One is the most common, and it is harder to find pins with 4 stars.
Some of the patriotic themed jewelry you will find include the flag, flag colors, the eagle, liberty torch, V for victory, and the different emblems of the different military branches.
When looking for sweetheart jewelry search for more unusual or rare- especially GI made. It is always nice to find the commercially made items with the original tags or presentation boxes. Matching sets are harder to find. Some commercial designers known for producing beautiful patriotic jewelry during WWII was Trifari and Walter Lampl.
A really good reference book is "Antique Sweetheart Jewelry" by Nicholas Snider.
Like all vintage jewelry, it is the stories and history behind the pieces that make them so special.