Here's a really pretty pin!
This is a crystal Chaos Emerald pin. Atop the metal back is a cut crystal in the shape of a chaos emerald. Each crystal is colored like a different one of the 7 emeralds. So what's the problem?
You likely can't ever collect them all, unlike Sonic. This pin (and it's little framing card you see here) is a part of the "Loot Box Gaming Box" trend where if you subscribe, you get a randomized collection of themed items each month. In January 2018's box, people got a pin, but the color (of course) was randomized and you only got just a single one. They weren't sold anywhere else. Still, it's quite nice! Here, it's displayed at the feet of the First4Figures Tails Collectors Display figure. (So, it's pretty big). 

TODO: Show other colors