I found an interesting book The Crystal Sun by Robert Temple on finds
of ancient lenses and of glass and rock crystal spheres used as
magnifying glasses and for illuminating text and other minute work.
It seems the ancient view of the sun was that it was a body fixed on
the outmost sphere (or heaven), not producing, but concentrating the
light of the universal fire beyond that sphere, in other words a type
of crystal sphere.
Herodotus uses 'hyalos' for "rock crystal" (III, 24). According to
Temple, Paul Jablonsky (Opuscula, 1804) believed the word to be of
Egyptian origin. But I thought I recognized *sH2w-l/n- "sun" (> *swal-