Small stone medallion - "CRISTO" inscription?

Anyone know what this might be?
pics:
https://dl.dropboxusercontent.com/u/3979840/cristoPix/index.html

I found this little artifact in Paris in the Tuileries Garden. What caught my eye was the inscription "CRISTO" and the distinctive shape.

Looking at the pictures, it is a partial piece of a small discus-shaped object. There is a hole in the center that has a slight chamfer on one side. I estimate that the whole unbroken piece would be about 1.6 inches in diameter, and .2 inches thick at the center.

The lettering looks suspiciously modern, but what could it be? "Cristo" is "Christ" in Italian.

Any ideas? Old? Modern? What is it?!