From http://www.windycreek.com/weatherrock.html but there are lots of variations.

This rock is the perfect weather indicator, it never fails. It is more accurate than your local weather person. This rock is the living word and is 100% correct. This is how it works:

a dry rock means fair weather.

a wet rock means it's raining.

a dusty rock means a dust storm.

a swaying rock means it's windy.

a shadow under the rock means it's sunny.

a white rock means it's snowing.

if the rock is jumping up and down, an earthquake is upon us.

if the bottom of the rock is under water it's a flood.