This is one half of a fractured geode which shows a much earlier fracture that was healed by quartz crystals filling the earlier fracture. While it is very common to find geodes with fractures that are filled (or healed) by secondary deposition of silica in the crack (fracture), it is quite unusual to find that the secondary deposition results in macro crystalline (visible to the eye) quartz crystals extruded from the crack. To me it looks like a crystal quartz zipper! Rock #3998