I have to bow to the expertise of geologists here, as I personally have no idea what this is ,other than there is an abundance of them scattered in front of Icelands many glaciers . I’ve been informed ( i’ll get letters !) that this is type of stone is known as a Tachylite, a rock formed as it was hurled in its molten state against the roof\side of a glacier while still in its molten state .The shock of which, creates fractures throughout the body of the rock, creating these amazing 3D puzzles for us to wonder at , Ive got one of these in my shed !
This makes these objects quite unique as this as far as I know doesn’t happen at least in the present day ,anywhere in the world.
Certainly it will make a talking point hanging on your wall (Shameless advertising there !).