A Ninja Star? A mutated Pog? No! This is a pin or badge. The sticker is holo-foil type, and the badge itself is plastic. The 3D rendered head is a design exclusive to the UK (somewhat fortunatly, as it was early in the 3D era)

This was a give-away of the Fleetway Sonic the Comic book. They often had little trinkety items attached to the book. Info credit to Smindas.

Here is a "Sonic Channel" button style pin. This features Adventure Style Sonic with a nice background fade. Aquired at a flea market, it was likely a promotional item...but for what?

These are collectable buttons from a store in England called "Hamleys". You could only win them in one of those arcade machines that dispenses prizes. Naturally, this increased the difficulty of getting one! So they're pretty rare.
There were 4 different ones in all. Other photos pending. notice it says "Professor" Robotnik and not "Dr". Why? Credit goes to MCRG for these!