Bismuth - is it an element? Metal? Man-made crystal? It’s all that and more! One of the easier metals to work with, these geodes were homemade by an enterprising dealer by pouring molten bismuth into eggshells, pulling out the ‘skin’ as it hardened, then peeling off the shell at the end. At least I think that’s how they did it.
At any rate, these are lovely and mysterious crystalline forms, sold with a sturdy iron egg stand, the better to display them right away.