I think Nick is on the right track here, but it looks like it may be a bit simpler than what he's proposing.
You can buy epoxy resin hobby kits that are used for casting molds; I remember making grape clusters when I was a kid. You can buy all kinds of different dyes to obtain whatever color you want. Basically, you mix the two parts of the epoxy, add in the color you want, then fill the missing triangle in your belt buckle. Once it dries, it'll be there permanently. The only part that will take some experimentation is getting the color matched; that will probably take quite a bit of trial and error. Here's an example of the kind of craft kit I'm talking about.
[link|https://www.amazon.com/LETS-Coaster-Starter-Everything-Include/dp/B0987H729P|Amazon.com: LET'S RESIN Coaster Kit, 16oz DIY Epoxy Resin Kit with Molds and Mica Powder Complete Set, Crafts for Adults, Beginners, Ideal Starter DIY Art Supplies Gift Set for Adults]