There are many different material can be used to make buckles, below picture will help you to know it better:
Stainless steel is mostly used for different buckles. There are some different types of buckles:
1. Hook Buckle
2. Needle Buckle(Also called Tang buckle)
3. Bracelet Clasp(Or Jewelry clasp)
4. Push-Button Hidden Clasp(Also called Butterfly Clasp)
5. Hidden Clasp
6. Leather Deployment Buckle
7. Folding Clasp with safety
Below are some pictures help to have a better idea about them.
Above might not have included all buckle types in the watches world.