First, I tried to poke out the black insides. You're right, it felt like sandy clay, and no amount of poking had much effect.

Okay. So how to clean the gray crap off? I took Han's warning about how it might be fungus.

So I put a few beads in a bowl and added boiling water.

Yay! All the white stuff is GONE!

And the black insides now look a lot better.

Another batch in progress this morning.
Thanks again, Hans!