Iceland spar, Helgustadir

Iceland spar is optically clear calcite, and it did more for the history of optics than any other crystal. Lay a piece over a printed line and you see two lines. The crystal splits light into two rays that travel through it at different speeds, an effect called birefringence, and calcite does it strongly enough that you need no instrument to see it.

That property is why the mineral carries a country's name, and this is the ground the name came from. Helgustaðanáma, the mine on the Helgustaðir land about seven kilometres east of Eskifjörður on Reyðarfjörður in eastern Iceland, was worked from the middle of the seventeenth century into the twentieth, with the industrial phase running from 1855 to 1925. For most of that time it was the world's principal source of optical-grade spar. It produced the largest and clearest known specimen and most of the Iceland spar sitting in museum cases anywhere.

What was done with the material is the reason to care. Clear calcite from here is what let seventeenth-century natural philosophers argue about how light behaves at all, and it is the raw material of the prisms that made polarised light practical to produce in a laboratory. Polarised-light microscopy grew out of that, and with it the whole of modern mineralogy and petrology. Every geologist who has ever looked at a thin section is downstream of this hole in the ground.

Now the part that matters if you are going. Helgustaðanáma was protected as a NATURAL MONUMENT in 1975 and collecting there is strictly forbidden. Not discouraged. Forbidden. The rock formations are protected too, so no hammering.

That is a fair trade, and the reason there is anything left to walk up to. There is a marked hiking route to the workings from the Eskifjörður road. Go, look into the pit that gave the world polarised light, photograph it, and leave the calcite where it lies.