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I have acquired two pieces of Icelandic Spar (optical calcite) with surface and internal fractures that should be serviceable as sunstones.

I have observed the double refraction and cancelling double refraction, a rainbow effect, and varying intensity of stuff. I stood myself in the shadow and looked through the stones (one at a time) at the sky. I observed several effects including the second image rotating around the primary, but everything seems to track with the rotation of the stone rather than the position of the sun.

I tried but failed to observe polarization cancellation from opposed polarizing plates. I was expecting this to be the answer but could not observe this effect.

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  • $\begingroup$ I'm not sure this is actually a physics question - the Wikipedia articles suggest that the phenomenon used to locate the sun is Haidinger's brush (and the fact that the polarization produced by scattering is largest at 90° angle to the sun), which is difficult to perceive for many people with an untrained eye. So maybe you just need to train to see the brush. $\endgroup$
    – ACuriousMind ♦
    Commented Jun 10, 2019 at 11:02
  • $\begingroup$ @ACuriousMind You could write that as an answer and I could certainly check it. $\endgroup$
    – Joshua
    Commented Jun 10, 2019 at 14:16
  • $\begingroup$ @ACuriousMind Wikipedia's discussion of the Physiological causes of Haidinger's brush is hard for me to understand; "attributed to the dichroism of the xanthophyll pigment... Pursuant to the Fresnel laws, the behavior and distribution of unguided oblique rays in the cylindrical geometry of the foveal blue cones produce an extrinsic dichroism." This is certainly an attempt of a physics-based explanation of the brush effect but I think it requires some orientation of the pigment molecules for a complete explanation. $\endgroup$
    – uhoh
    Commented Feb 3, 2021 at 4:44
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    $\begingroup$ @ACuriousMind That said, we then need to understand what role the calcite crystal plays in using Haidinger's brush; if the polarization is strongest at a 90° angle to the sun, why don't those who can see Haidinger's brush just use it directly? Doesn't the particular way that the calcite crystal participates in this have some physics in it? Isn't there a physics-based answer possible here? $\endgroup$
    – uhoh
    Commented Feb 3, 2021 at 4:44
  • $\begingroup$ Another thing you might like to try is to look at the Moon through your sunstones when it's in the First Quarter or Last Quarter phases. At those times the Moon is 90° from the Sun. I've never tried it with calcite, but the effect is quite noticeable with polarising filters, eg Polaroid sunglasses. The general sky polarisation is easy to see with a polarising filter, but it's a nice effect with the Moon because it enhances the contrast significantly. $\endgroup$
    – PM 2Ring
    Commented Feb 3, 2021 at 6:03

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Half an answer. I got it working but don't understand it yet:

I spent another half an hour outside trying this way and that to get something out of my sunstones. I got a double-banded rainbow effect when looking through either stone that moved with the stone (definitely not Haidinger's brush) and rotated with the stone, but its brightness depended on the orientation of the stone with respect to the sun. If I turn the stone I find a place where one of the two sets of bands disappears. Once so turned, the top and bottom edges of the stone point at the sun. This works only if the stone is backlit by the sky, and critically, if the stone is not directly illuminated by the sun.

So now it's a pure optics question. I don't understand the rainbow effect; with all the edges forming parallelograms, I wasn't expecting a prism. The brightness has got to be polarization cancelling but it's cancelling something I didn't expect to see at all.

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