Jelly Belly is often used to describe colorful cabochons and moonstones, but the official definition of a Jelly Belly piece is one that is made from thermoplastic acrylic resin (Lucite) and placed in sterling silver or a different form of metal. A true Jelly Belly, therefore, is clear. A more relaxed definition allows for glass middle pieces which is why some people have a broader definition of this piece. So it is also known for its color partly because the middle resin area is the highlight of the piece and many sharp neon green or pink centers can be found for various jewelry items.
Designs took shape in the form of animals, with sterling or metal as the frame and the Lucite occupying the middle, giving a flashy color and design to an otherwise plain piece. Perhaps the two most common types were roosters and penguins. These types of designs lent themselves well for pins and brooches because they were eye-catching and the silver frame allowed for a strong base. Other symbols created were flowers and sailboats, with transparent Lucite for petals and sails.
Costume jewelry manufactures loved to use jelly bellies. Because of a cheaper cost, they were able to create multiple sharp designs that would go well with virtually any type of outfit. The outside of the jewels could consist of rhinestones or moon shells, hence why Jelly Bellies are commonly associated with these types of materials as well. The advent of costume jewelry probably widened the laymen definition of the Jelly Belly because of the number of different items that could be reproduced for a cheaper price.
The Jelly Belly was made popular in the 1940s, during World War II. Sterling was expensive at the time so using Lucite as a design element was a brilliant way to make jewelry more affordable and still maintain its appeal for the masses. One account also claims that companies like Trifari were able to get their hands on expensive and higher-quality materials precisely because they were able to get their hands on unused Plexiglas windshield and turrets from the war effort and turning the pieces into Jelly Belly jewelry. Lucite allowed for creativity and loud and flaunting designs. The actual technology to develop a Lucite composite was invented in 1937 by DuPont so this type of jewelry still remains relatively (when compared to forms that have been around for centuries) young.
(photo credit) http://ww.morninggloryantiques.com