Ok, here is a really wild guess, mostly because I have no idea of the geology of the area you found it in.
1. Petrified wood. The exterior doesn't look too "woody" but that doesn't mean much. Many times, a "hollow" area can be replaced with a different kind of silica (agate, or quartz) than what the wood was replaced with.
2. A piece of a hollow lava bomb or small tube, later filled with agate (suppose it could possibly be opal in either option 1 of 2). The huge amethyst geodes from Brazil are found in lava flows and seem to be air bubbles or spaces that have slowly been filled with quartz. Occasionally, they are found solid.
3. A geode. Filled solid so the crystals are either incorporated into the massive form or filled so fast crystals did not get a chance to form. Does't look to "geodey" though.
It is hard to see the interior rock. Does it look like agate, quartz or maybe opal?