Your sample is very simular to those I find all the time. Conditions are favorable for these types of stones to have crystals grow in and on them.. thou I can't say that its a speical type of rock you have.. Just conditions are favorable for the crystals.
However, if the stones looks (which I really can't see in the image) to have a milky build up (mostly agate based) then it could be a chalcedony/agate/quartz mixture.. and it could have attempted to turn into a geode or thunder egg.. Make a note of the crystals.. rounded or pointed?..