Asteroids appear as small rocks, floating in space. I'll add images later for an example. Please see the faction message boards for scouting reports, these will show all asteroids in systems we actively scout.
Expanse asteroids tend to be MONSTROUS in size, with huge amounts of iridium as well as the ore in question.
Asteroids are different colours.. matching their respective refineries.
Fullerite - Green
Gold - Orange/yellow
Iridium - Brown
Quadrium - Maroon/purple
Tetrium - Grey
Zirconium - Blue