I love vintage reverse carved Lucite jewelry, especially the little brooches from the 1940s and 1950s. Lucite is a trade name for acrylic thermoplastic (Perspex is another name), and it is a fabulous material which can be melted down and re-used.
As well as being factory made, reverse carving Lucite was a poular home made craft hobby during the mid 20th century. Morning Glory Antiques has a superb article on how it was created, complete with images of a 1947 magazines step-by-step DIY guide on how to carve the plastic
The reverse carving was finished by painted the inside using acrylic paints, oils or water based colour pigments (which is why you should never get them wet as the paint sometimes dissolves, as early on my my career I found out to my horror!)