This is the back of the metal disc, it's shakudo ( an alloy of copper and a small amount of gold ) This is the alloys characteristic black patina. It's really quite tricky to capture the depth of colour in a photograph so I've pushed the settings in photoshop a bit to better show the contrast. In reality it is a solid glossy black. The moth is depicted in the finest gold inlay but I'm not letting on how I do that bit just yet. The signature, or mei, reads; Fo oo do; Walking in the heavenly/kingly road. It's the go, or art name my teacher Izumi Koshiro, gave me 16 years ago when I first went to study with him in Tokyo.
I don't always sign my work in this way but on things that are more defined by the Japanese tradition it just seems to me to look right.