"This morning I saw a thick swarm of diamond dust precipitating from ragged batches of stratus in the sky. The crystals were really big sectored/stellar plates, in the order of 3 to 5 mm in diameter. There was extreme glitter in the air!

At first the elliptical halo manifested itself as an evenly lit disk that had no distinct "arc" in it, just the blindingly bright disk. It was so bright I was at 1/8000s, f20 at times in order to reveal the disk close to the Sun.

Soon the disk morphed into a distinct elliptical halo! At times there was also a circumzenithal arc in the sky but no other halos whatsoever. Presumably the sectored plates had the necessary 90 deg wedge in them, along with the exotic wedges that produced the ellipses."

All images ©Jari Luomanen, shown with permission

Elliptical halos are usually small and they are rare.

They are argued to result from sun rays being refracted slightly by passage through plate-like pyramidal crystals.    However, the very shallow angles that are necessary are unphysical. Furthermore, ray tracing simulations hardly ever accurately reproduce the multiple arc positions or relative intensities.

Larger sectored crystals, as imaged by Jari Luomanen, and snowflakes might have a wedge shape tapering in thickness from the centre.   Crystal faces might also be curved. These might generate elliptical halos.

And an even rarer filled ellipse?   A range of crystal wedge angles?

The most certain thing we can say is that how they form is uncertain.