Rainbow Boulder

Every other boulder falls and grinds along the floor. This one launches into the air and flies, passing straight through terrain and lighting up as it goes. For about a minute it hunts on its own, hitting enemies hard and drifting toward buried loot the way a fairy does. Its own music track plays while it is out, which is the easiest way to tell one is still active.

It shatters the instant it touches a boss. That rules out the one thing boulders are famous for, parking a stack of them over a worm boss and letting gravity do the work. Use plain boulders for that and let this one clear ordinary enemies while you dig.

You can craft and place them yourself, and a placed one behaves exactly like one you stumble into. The glow makes it readable in an unlit cave, so it doubles as a scout: watch where it drifts and there is usually something worth mining there. Only ten can be active at once and anything past that quietly fails to start, so setting off a whole room of them together is wasted.