The Amazing Crystal Garden pack contains limestone pieces which, when placed in distilled white vinegar (available from all supermarkets, not included in the kit) and the instructions followed, produce real crystals similar to those shown below.
Food dyes (not included) can be added to the vinegar to create coloured crystals. Limestone is calcium carbonate and it reacts with the vinegar (acetic acid) to form Calcium Acetate. Historically called acetate of lime, Calcium Acetate it has a number of uses including as a food additive and in the manufacture of Tofu.
Completely safe for children when instructions are followed. Supplied in a merchandiser unit containing 48 boxed crystal garden kits.