When you get to Eskifjörður, if you drive through the town and along the gravel road that leads up the coast, you will come to a signboard with some information about the Helgustaðir mine. Drive only a little further, and you will see this:
Follow the rubble up the mountainside for a short while, and you will come to the mine.
For about 200 years the mine yielded large quantities of calcite crystals that came to be known as "Iceland spar".
The mine is now protected, and it is forbidden to remove crystals from it, but there are still plenty of them around:
Most are not clear enough to show the crystal's ability to refract light.
A short history of the mine in English.