What kind of matter are black holes made of? Imagine a black hole originally formed from, for example, Rubidium atoms. On the other hand, one made from, for example, Helium atoms. Will it be there any difference between the two? Or perhaps once formed it all turns into the same type of matter?
Can the "composition" of the matter-energy in the black hole be inferred somehow?
Could one say that "black hole substance" is a distinct form or state of matter different from other forms or states of matter?
 A: A not-entirely-silly answer: Black holes seem to act like a very, very heavy fundamental particle, described only by its mass and quantum numbers like spin and charge. Those numbers just happen to be very large compared to typical fundamental particles.
If you made them out of different kinds of atoms, they'd have the sum of the quantum properties of those atoms. If you're using neutral atoms, the black hole will be neutral, but if some of the atoms are ionized, you'll end up with a positively charged black hole. If you use entirely atoms that are prepared in a way that makes them mostly spin-up, the black hole will be spin-up. Most of the information about what you put in disappears, though.
A: Assuming they have same the mass, net electric charge, and angular momentum any two black holes will be identical. There isn't any way to analyze a black hole and figure out what kind of atoms have crossed its event horizon.
We do not know, in terms of empirical data, what happens to the matter that crosses the event horizon, because no information about what is happening 'inside' the black hole can cross over the horizon into our part of the universe. Presumably, it arrives very quickly at the singularity, separated by tidal forces into a stream of fundamental particles.
