Side view of a "crystal garden". The long coloured streamers are the crystals themselves. The medium in which they grow is a saturated solution of sodium silicate or water glass. A number of different crystals are growing in this garden (not all of which are visible here). Each type has a characteristic colour; copper nitrate (light blue), cobalt chloride (dark blue), ammonium iron (III) sulphate (brown), magnesium nitrate (white) and iron (II) sulphate (green). Crystals form when a saturated solution of a suitable substance is either cooled or has some of its solvent removed by evaporation. The growth usually starts around small introduced seed crystals.