I recently learned about using mussels for buttons when I visited the Mississippi River Museum in Dubuque, Iowa and have been wondering since: can Zebra Mussels be used for buttons? That would create (even more) economic incentive to go after them.
These little fellows are, in general, small. I guess they can get 50mm (2in), but most aren't that large and they have thin shells.
Further, I'd be somewhat afraid that creating products from them would spread the invasive species even further. The professor I worked for studied them because of their invasiveness - the lakes he set traps on were obviously spread by people. They spread easily by the water in boats - microscopic young means people don't know they spread them.
The trick to identify genuine shell buttons is to to touch one against your upper lip. A shell button will feel cool to the touch, a plastic one won't.
This has happened many times throughout history.