Mob protecting geodes

Geodes are interesting as features, but I think they should have a little 'challenge'. I thought of a unique mob that can only be found in geodes, each geode will only have one. This mob could be a construct, or maybe some sort of ghost. The geode guardian will attack the player when he breaks a block belonging to the geode. Obviously it would be a very strong mob, even a miniboss, once defeated the player will be able to use the geode freely