I have a nice geode sample that is sadly structurally unsound, and I am looking for a solution to make it sturdier so I don't run as much of a risk of it breaking on me. I have a separate geode that has a black material on it that seems like it makes said geode more structurally sound. What material is it? I have tried googling to find an answer, but I have been unable to find a proper answer. I am looking to figure out what it is so I may get some to use on my other geode.
The material is rather sturdy, heavy, and cool to the touch. The lower "mass" of the dark stuff feels like concrete, though I can not tell what the coating on the outside is made of.

