The technique is exquisite. You let the sun and gravity break off a bit of a glacier, you soak it for a few days in salt water, then cast it up on a beach of black volcanic sand. After a night of the waves splashing sand all over it, it sets in the sun. It’s really fun to chase this art form down,. Here’s a troll with a monk in its belly, holding Christ as a child. And isn’t the Mjalður the Bell Ram off to the left? Why I think it is.
If you haven’t read Gunnar’s Advent, it’s time.
Of course, you could just go right to the source, though.