This is very, very cool. Geologically, there are ways in which minerals move around and get deposited with rock. A common phenomenon is for a crack to form due to cooling of molten rock or an earthquake or something, and then this space gets filled in. Stuff might just fall into it. Liquidizer rock (magma) might intrude into it. Hot gasses containing residue might build up a deposit within it, or liquid water might flow through it leaving behind minerals, which fill the crack. The thing is, geologists have studied these processes and have a pretty good idea of what they are, how they work, and what they look like.
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