What is this?

I’d be interested to know if somebody can identify this (it’s a little larger than a squash ball:

UPDATE: The prize goes to Carl:

It appears to be an air stone for fish tanks/ponds.

Since yesterday evening it found its way through our rear windscreen:

I’ve held back on the vitriol, but if I could get hold of the person responsible, I’d happily introduce their head to both the missile and the glass.

Tags:criminal damage, scum, smashed, windscreen