I try to scratch the surface with a pocketknife, and it laughs at me. Not even a mark on it. The stuff on the inside is softer, able to be scratched slightly and chipped by the tip of the knife.

The vinegar thing was a trick I learned when I was young as a field identification for limestone. The acid reacts with the limestone and produces carbon dioxide bubbles.

Perhaps more background on the area where this was found will be helpful. In Wyoming there is a rather large area where a specific type of petrified wood occurs called Eden Valley. The reason this wood is unique is that it was submerged in water and encased in algae before it could rot. As a result, the petrifaction process created much opal, basically silica (quartz) that formed under water. I suspect the shell of this specimen is opal, just not a precious type, because the material resembles many other pieces I recovered from the same area.