
A quick search online identified it as Nielloware. Specifically it is vintage sterling silver (c. 1940s, Siam). The process involves first engraving the silver design, then carving out background areas which are then filled in with a sulfur power mixture. The piece is baked at very high temperatures and then polished. The final step is to re-engrave the silver. In Thailand it is call khruang thom. This process dates back hundreds of years, and was especially popular in the 13th and 15th centuries in places like Russia, India, Japan as well as Thailand.
Nielloware became popular in the US from the 1950s to 1970s when it was introduced by US servicemen during the Vietnam war who sent jewelry home to their sweethearts. My father was in the Airforce during this time, and no doubt how it ended up in my mom's jewelry box.

For me this piece of jewelry reminds me of being a young girl and sneaking into my mother's jewelry to marvel at the things inside the box.