1How to craft it
Heavy Assembler
it falls on you
These obey gravity the way sand does, and the ocean stacks them in large clumps, so cutting into the underside of one drops the whole thing on your head. That hurts, and an early character standing under a tall pile can lose most of a health bar to it. Dig at it from the side or from above, never from underneath.
what it is good for
An Extractinator turns it into plain sand, which is a tidy way to empty out a stack you only mined by accident. Selling is not the answer either, since the loose shells the block is drawn from are individually worth far more than the block itself. Treat it as decoration for a beach build and nothing more.