Reddit mentions of Geodes: Nature's Treasures

Thirded for probable non-geode.

As an aside, Northwestern Mexico is a major geode producing region. The miners, working in tiny, poorly lit tunnels, fill burlap sacks with geodes as they pluck them out of the walls -- their pay is based on a 2-bag-per-day quota, with a bonus for filling extra bags. The sacks are hauled up on a janky-looking pulley system called la cigüeña ("the stork") and trucked to a facility where they are first sorted by size on a bizarre wire contraption. Workers then heft the rocks by hand to sort out the valuable hollow samples from the solid ones, which look identical but are essentially worthless and just discarded in a giant pile outside. (Source: Zeitner and Cross's Geodes: Nature's Treasures)

I can tell you for darn certain that I wouldn't be able to tell the difference between a hollow and a solid without cutting that sucker open!