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.

Completely safe for children when instructions are followed.