This beautiful artificially Coloured Calcite Geode - Black Rock - Yellow, offers its colour once opened.
Geodes are hollow stones, inside which crystals have formed. Each geode is therefore absolutely unique, and the result of a very long process of crystallization. 
Geodes can be difficult to find, as they are difficult to recognize on the outside and are lighter than solid stones.
 
NB: Geodes and crystals inside them are painted.
The average size is 8.5x6 cm and might vary as it is a natural product.