Snow Globe is right. Mica is a rock that was used in handmade marbles and a very small number of experimental newer machine mades. This is probably undissolved potash or something like that.
Correct on the banded lutz. They call them opaque for some reason even though they are translucent. Condition keeps it from having significant value IMO. Get another opinion since handmades aren't my forte, but to me this would be under $50.
2-3 thousand marbles sounds like a lot maybe to a new marble person but it's a very small sample to draw from for different examples. As mentioned at AAM, it's best for you as a seller to not try to make all these common marbles into big financial wins for you. The people who are doing free IDs for you are also the very same customers you are trying to sell these to for inflated prices and it doesn't settle well. Just FYI. From our perspective, you just put 4 marbles like we have in our junk jars up for $100.
If you just want marbles for decorating or in a fish tank or something, give what they're worth for that. As far as collector value, I don't think I see anything I would want at all. Even for free. They might have some learning value if you're wanting examples to learn from.
It has a flaming pattern to me. I'd call it a flame, someone else wouldn't. It's up to you on this one. Some aren't what we'd call flames, some are very obviously flames, and some like this are in the grey area where the two meet.