Life Crystal Boulder

It looks like a Life Crystal set into the wall, and it is a boulder wearing that sprite. Swing at it and it drops, then rolls, then flattens you. In the seeds where these generate, treat every crystal as one of these until you have checked.

The real thing gives off sparkle particles and lights up under a Spelunker Potion. This one does neither. It shows on Dangersense instead, like any other trap, which makes that potion worth more than usual while you are exploring one of those worlds.

The reward is genuine: when it finally breaks it leaves a real crystal behind. The safe method is to give it somewhere to go. Dig it out from above or from behind, let it run down a corridor into a wall, then walk over and pick up what it dropped. Mining it while you are pressed against it means eating a full hit, because the damage does not scale with how fast it is moving.

They sit on flat furniture the same way ordinary boulders do, so a table of them wired above a doorway works as a trap. Unlike normal boulders they will not stack directly on each other, which limits how much you can pile into one spot.