<i>You mention granite, granite often contains crystals of quartz and feldspar that would catch the sun and perhaps radiate ..or am I going too far with that one?
Many circles contained scatters of quartz chippings or light quartz sands which have been interpretted as ritual deposits. </i>

I don't think you're off beam with radiating in the sunlight. Grianchloch, Irish for quartz is Stone of the Sun. Some granites when in sunlight look as though they're covered in glitter, I have a bit on the windowsill and for most of the time it looks like a lump of stone but when the sun catches it just right, it looks as though it has something like a bright silvery (moon?) light flowing over it.

Local materials were used where possible but other stones were brought from other places, too, seemingly for their different colours or their particular striations, often quartz 'stripes'

Rune