Rocks could be potential sources. Crystals that large are by no means rare, with feldspars being the most common on Earth and perhaps on most rocky planets (quartz is well known of course but I think would be rare without the magmatic fractionation that happens due to plate tectonics, which is perhaps unique to Earth in the solar system.)

Volcanic glass (eg obsidian) is also shiny and by no means rare in the solar system.

Many asteroids are also metallic, and perhaps metal crystals or fracture planes could produce reflectors of the right size.

But maybe it’s just aliens.