Brooch

It amazes me that every time I go into the garden there are new pieces of glass on the surface of the soil and every time I dig I unearth more nails and screws and other bits of metal. I was carrying some indoors today when I spotted an unusually smooth curve of metal protruding from the ground. I bent down, picked it up and scraped the soil off. Since then I have brushed it with an old toothbrush, sprayed WD40 on it to loosen the catch on the back and polished it. Now I need to find who it belongs to.

(It's a bit less than 6cm across, not the shield it might seem to be in this image.)