A floating rock!

During our walk on the beach yesterday the eagle-eyed youngest Talpid  picked up an unusual rock the like of which I had never seen before.

It is, I think, vesicular lava, derived from volcanic magma full of gas bubbles. 

The Talpids were surprised to find that this is a rock that floats in water! There are volcanic rocks along the Aberdeenshire coast but the romantic in me likes to think that our rock had floated across the  ocean from Iceland.