That's a pretty solid idea. I like the concept of an accessory that rewards you with extra minerals while digging.
That said, I do have a concern. If I'm a player equipping this item, it might be too easy to exploit. For example, I could just weigh down the S or Down key with something heavy and use a simple autoclicker on the "Back to Top" button. After a couple of hours away, I could come back to 999 copper, 999 iron, 500 gold, and probably 150 titanium ores. That kind of passive farming could unbalance the progression pretty quickly.
Still, I think the core idea is great. If we could tweak it slightly to keep it from being too abusable, then it could be added to the game.