This Micro Cache Stone - Black/Grey, is quite realistic, and a great way to hide your keys, and make an even better cache.  They are not water tight, but if you put a log in a ziplock bag it will keep it safe and dry.  You can even respray the rock to match the area it will be living in to make it harder to find.

They are not supper light to pick up so they won't stand out when placed well, they have a plastic insert for a base which fits tightly, but it is not waterproof.

The internal compartment measure approximately 6 x 3.3 x 2cm  (length x width x depth)

The external rock measures approximately 8 x 7cm