From my Great Aunt Lillian's couture design collection, this great 2-part belt buckle is made of celluloid. Or, at least that's what Aunt Lillian told me years ago.
Her sister-in-law, Martha Sleeper, was well known for her acting career in Silent Films. But, better yet, Martha is even better known for her Bakelite jewelry designs. Martha's designs were much more whimsical, so we can be pretty sure that this was not one of her designs.
The reason we tell this story: Aunt Lillian knew her plastics. This doesn't feel like Bakelite, but we're pretty sure it is celluloid of some sort.
The color is mottled olive green (almost Army green) with some complementary colors in the marbling. Atop the triangle-shaped plastic pieces, are metal triangles filled with rhinestones. This has been repair, but the repair alone was made over 50 years ago. Today such a repair would have been made with glue. You can see the repair from the back.
The buckle also features raised scallop on the larger end of each triangle. These have been carved into flower shapes.
When hooked together, the belt buckle is 3-1/2 inches wide. The skinniest height (just in the middle where the two pieces connect) is just a smidgen under 1/2 inch. The wider part (on each end) is 1-1/4 inch.
The buckle hooks underneath with a metal clasp (one side is a hook and the other a loop). This is great because you can't see the hook from the front. There is some rust on the hook's metal. We never clean vintage items for fear we would ruin the value or memories behind it.
Each of the two halves have two rings for attaching them to the belting material.
This is quite a find!