The Amazing Crystal Garden Kit

https://www.youtube.com/watch?v=3TOEkHO4c-Y

The 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.
 

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.