Iceland spar as "mythical sunstone"?

Via Matt P, Nature News has a story about some Swedish scientists who think the Vikings might have used light-polarizing crystals like Iceland spar to estimate the location of the sun on overcast days. Apparently there's a bit in one of the sagas where a character Olaf "grabbed a sunstone, looked at the sky and saw from where the light came, from which he guessed the position of the invisible Sun." So maybe Olaf had a block of calcite in his Viking-precursor-forerunner-of-the-pocket:

Scattering by air molecules in the atmosphere causes sunlight to become polarized, with the line of polarization tangential to circles centred on the Sun. So Ramskou argued that by holding a crystal such as calcite up to the sky and rotating it to check the direction of polarization of the light passing through it, the Vikings could have deduced the position of the Sun, even when it was hidden behind clouds or fog, or was just beneath the horizon.

The obvious -- and probably fatal -- objection is that there's no real evidence the Vikings actually had sunstones; the sagas are not reliable on this sort of issue. But this story sounds pretty plausible to me; looking at the sky through bits of transparent stuff is the sort of thing one would do.