Why does ulexite (a.k.a. tv rock) refract a laser beam as concentric rings?

I have a piece of ulexite mineral, and while playing around with a laser I found that if a laser beam is pointed at the rock parallel to the fibres the light gets refracted as two or three concentric circles. The radii of the circles increase as the angle between the laser and the perpendicular of the surface increases. Then, if the laser is pointed perpendicular to the fibres, the light spreads on the plane perpendicular to the fibres, with similar intensity in all directions.

Shining a laser beam on ulexite, parallel to the fibres.

Shining a laser beam on ulexite at some inclination.

There is a paper about this exact topic ("The strange optical properties of ulexite" by Profs. Garlick and Kamb) but I found it hard to understand (I don't know a lot about optics), and doesn't explain what happens when the light is perpendicular to the fibres. Some important points seem to be that ulexite is made of fibres surrounded by lower refractive index material (essentially a natural optic fibre), and that it is biaxial.

As far as I can tell, this phenomenon is not a result of interference or diffraction, which were my first thoughts on seeing the pattern. Birefringence might be part of the answer, but it doesn't explain the rings by itself (I pointed the laser at a piece of calcite and the result was very different).

Could it be related to conical refraction? The diagrams I've seen for conical refraction suggest the light beam comes out of the crystal as a cylinder, while in the case of ulexite they're cones, not cylinders (the radius changes depending on how far the screen is), but maybe something similar could happen?

Could someone explain this phenomenon?