Fabulous Georgian (early 1800s) yellow metal and silver buckle brooch with beautiful large round natural moonstone cabochons. The back is silver - probably a lower grade than 925 silver, eg 800 parts per 1000. I have not tested the gold tone yellow metal, but think it is could be 15ct gold or plating, as was often used, given the age and quality of this delightful little pin, though it could be just a yellow metal or a finish such as pinchbeck.

Many listings of brooches like these say that these pieces were originally Georgian shoe buckles, kept in pairs. This piece fastens with a simple C hook clasp and pin which may well have been added a little later when buckles went out of fashion. The brooch is in good vintage condition - the clasp ends are a little visible under the two end cabochons, which is why there is a little darkening for those ones in appearance. 

Here are some other listings of similar brooches: