Silfurberg

When I was about 9 years old, my grandmother gave me a piece of Iceland Spar, it had belonged to my grandfather and she felt I ought to have it.

Iceland Spar

This is the piece, my kids have it now, somewhere along the way it broke into three pieces while in storage.

She showed me the birefringence, I remember playing with it for hours. I also remember having had it impressed upon me that it was important, not just a curio, though I am also acutely aware of the possibility that I have reinterpreted my partial memories based on what I learned later.

Birefringence in Iceland Spar.

The doubled image due to the different index of refraction of the two polarization states is clear.

I also have memories, possibly real, of having the piece at scout camp, probably when I was 11, in early spring, and looking at the clouds with it. I didn't see anything, in retrospect my eyesight had already deteriorated then and I couldn't have. I have memories of some older scouts saying "they could see it!".

Seen what?
Assuming my memories are not false?

'Viking sunstone' found in shipwreck - well, in the Sagas there are tales of the "Sunstone", not to be confused with the loadstone, and how it could be used to find the Sun in the open sea.
The records are not as sketchy as news stories make out, Documents relating to Iceland spar - Leó Kristjánsson at the University of Iceland has a good collection of references and articles. There was a "solarium" find in a Greenland settlement in about 1956, which lead to some interesting articles in Icelandic magazines, and in 1969 a danish archaeologist, Thorkild Ramskou, wrote a book on sunstones. So, it is quite plausible that the concept would have been actively discussed around me when I was of an impressionable age, it was a topic for well read people to contemplate.

In particular after the Sagas, church records show "solaria" as capital items, they were listed in the 14th century as a valuable, but it also seems clear to me the records confound several different items: some seem to be calcite or cordierite, one reference records the loss of the sunstone by an ignoramus who could not tell it from rocks on the beach (so not Iceland Spar, but possibly cordierite); others seem to refer to sundials, or stones carved with the cardinal directions, possibly for use with loadstones.

There is also a reference to a traveler in earlier times who used ravens to seek land, because back then they didn't know about sunstones... the Sagas imply a discovery and technological development path over the 9th-11th centuries, if one assimilates the context of different accounts.

So, in principle, one can use the polarizing properties of silfurberg and other minerals to localize the Sun's position on a cloudy day or at twilight, a handy skill if you are stuck in mid-ocean late on a cloudy day.
How, exactly, this is done is disputed, though (op cit) there are numerous speculations on how one might do it. The most plausible explanation is Haidinger's Brush - it would be consistent with the tales, and explain why it was a talent that few mastered and could be lost.

When, as an astronomer, older and more cynical, I heard of the legendary sunstones, my reaction was "duh, every child knows that...".
But that is not reliable.

However, my recollection is that in that scout camp several of us knew, through some oral osmosis, that you were supposed to be able to see the Sun on a cloudy day using that crystal.
I don't think any of us did.
We were on land, with familiar landmarks, we all knew the cardinal directions from that location.
Dropping us off in mid-ocean for a blind test was not feasible.

But I still have it.
I'm a theorist.

So, wtf, I took it out this afternoon and looked.

Now, it was cloudy, but the cloud cover was not thick though the Sun was covered. I rotated the crystal while scanning with peripheral vision, hoping to catch the yellow tint at an angle off the Sun, I couldn't have triangulated on the Sun from both sides, the sky was clear to the west, but it was worth a try.

I did not see Haidinger's Brush.
Somewhat to my surprise though I saw a very clear glint offset from the Sun by an acute angle. It was reproducible and photographed (see below).
The onset was sudden and only worked over a narrow angle and rotation angle.

Glint in the sky- the Sun is to the right, off image

This is a feeble reproduction of the glint I caught. By eye the contrast was sharper and there was a clear refraction rainbow.

So, what is it?
Well, it is pretty clearly coming from an internal fault plane in the crystal - presumably a complete reflection of one component of polarization of the light.
It gives a sharp response function at a quite acute angle, but what I don't know is whether the clouds were "thick enough" - I could have been seeing direct sunlight from a gap in the clouds.

I am somewhat intrigued, and may try it again if I can get a cloudy sky near evening with a clear line of sight to the horizon. If this works with serious cloud cover, that´d certainly do the trick.
For now though I suspect stray light contamination.
Good fun though. Excellent for illustrating some modern optics.

Oh, I have not seen the "Vikings", the new History Channel series, but I caught a few minute glimpse of the second episode this evening and ads for the first two episodes.
The "sunstone" they show is the wrong shape and colour, one of these annoying details that are so needless.
Looks like a fun docudrama series though. One of my bigger culture shocks was hearing tales of Ragnar Loðbrók from the English perspective after having read the Icelandic version...
However, it is rather unlikely he (in so far as he is not a composite character) lead the raid on Lindisfarne in 793, as he most certainly died in 865.