I came across this add on craigslist for "EXTREMELY RARE Museum Size Fluorescent Minerals & Crystals" but they look awfully similar to alum crystals to me.
Ceratinly artificial xls. Some of them may be synthetic "Alum" xls, some may be synthetic Cu sulphate (Chalcanthite) xls, some may be potassium chromate xls (which, by the way, is toxic). But all are lab-grown and artificial.