Alunite and various alums : laboratory crystallizations
For several years, very beautiful pseudo-octahedral crystals of potassium alum (abusively called alunite) pluri-centimetric, purple, green, red, yellow, blue, almost all colors exist (see photo below) on the stock exchanges and on the net above)... These are of course laboratory crystallizations which are in no way natural. These crystals can be floating or have grown on a piece of rock that is natural to it (usually gneiss or granite).
Alunite of course exists in nature, the crystals are much smaller than synthetic alums and also much less colored...
Chrome alums (photo opposite), potassium alums, etc... are chemically close to alunite, they are dissolved in water until saturation to allow the precipitation of crystals. Potassium alum in particular, available in pharmacies, makes it possible to carry out crystallization experiments at home, quickly and without danger.