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I've bought a calcite crystal and tried to see birefringence by shining a laser through it. I get a split spot in the centre of a sixfold pattern

enter image description here

The hexagonal pattern resembles a diffraction pattern, but I can't figure out what should cause it. The crystal is not perfect, of course, however the hexagon does not depend on the incidence point and very mildly on the direction. What can be the origin of this unexpected pattern?

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    $\begingroup$ How does the pattern behave as you rotate the crystal relative to the laser beam along various axes? $\endgroup$
    – Jon Custer
    Commented Mar 29, 2023 at 13:44
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    $\begingroup$ @EdV Yes! I agree! It makes much more sense. It is very likely that there are two single crystals stuck together, with a sort of twin plane.. $\endgroup$
    – mrf1g12
    Commented Mar 30, 2023 at 7:28
  • $\begingroup$ I've removed some comments which have been upgraded to a (partial) answer. $\endgroup$
    – rob ♦
    Commented Mar 31, 2023 at 4:04
  • $\begingroup$ To me it almost looks like they appear because of the internal reflections from the center glue region on the other facets. Do you see outputs in any other directions, besides of course of the ones present in the picture?? $\endgroup$
    – José Andrade
    Commented Apr 6, 2023 at 17:48

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As per the suggestion by moderator Rob, this is a partial answer to the OP’s question regarding the six spurious spots they observe when sending a red laser pointer’s beam through a calcite crystal. It appears that the ordinary and extraordinary rays are present, as expected, though maybe not well resolved due to the short path lengths.

My first thought was that the light intensity was too high, causing spurious reflections from the faces of the calcite crystal. Jon Custer suggested seeing what would happen if the crystal was rotated. Since these are easy things to test, I did so.

The figure below shows my simple setup: red laser pointer, low quality calcite rhomb, and scrap paper screen. The ruler provides the scale. Red laser and calcite rhomb 1

This replicates what the OP did, but only the too intense ordinary and extraordinary rays are revealed by the spots on the paper screen. No array of 6 extra spots. Rotating the calcite rhomb variously did nothing significant. So my working hypothesis met the fate most do: it failed.

So next I attenuated the laser intensity by a factor of 100 using a neutral density (ND) filter of optical density 2. The next figure shows the result. Red laser and calcite rhomb 2

Nicer looking spots for the ordinary and extraordinary rays, but no extra spots evident.

Another try, with my higher quality calcite rhomb and ND 3 attenuating filter between the laser and calcite rhomb. Red laser and calcite rhomb 3

No real change from the previous figure. At this point, I sent an e-mail to a friend (AChem) who is a high rep user at the chemistry stack exchange. He replied that “I think he does not have a single crystal. You can see a plane of bonded crystals. I don't know if that is causing artifacts.” He gave me permission to raise this possibility, so I posted it in a comment.

Here is the OP’s cropped image, with arrows showing how it might be two pieces stuck together.

enter image description here

The OP commented that this makes sense, but even if this is correct, what is the cause of the 6 extra spots the OP has observed?

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