My chin has 3 lava ledges in his cage and I see waste constantly. I think the white rocks turn more to dust than break up into peices (the lava ledges do both), so its probably just mixing in with the excess dust from their baths, making it harder to notice. Of course a little dust will get on their tounges and will be ingested, but I honestly doubt enough to cause any harm. I don't know if there's calcium, but if there is, the amount they end up ingesting (barely any at all), should not cause any problems. If you're worried, than limit it-give 1/2 the amount of stones you usually give, or a 1/3, but I honestly doubt it's necassary.
And I asked on Yahoo Answers before i joined this forum if it's possible for them to wear down their teeth too much since mine is suddenly devouring his one pumice ledge so fast. Everyone said there shouldn't be any problem with that and now I believe it. I checked his teeth last night and they look like a healthy length. I have a feeling that if it were possiible, they'd probably know to stop gnawing for a little while to let a little more of their teeth grow back in. I'm honestly no longer worried about that.
I also don't think they 'spit' it out. It just falls out. When my boy gives me kisses, his tongue isn't wet-I don't think they have wet mouths like us where tthings get stuck in there and eventually swallowed. Like if I'm being stupid and lean over him while he takes his dust bath and dust blows up in my face and if it gets in my mouth, it sticks and is swallowed
I don't think this happens for them-they're also smart enough to close their mouths, though, I'm sure, lol! Once my chin didn't close his one eye when taking his dust bath-that wasn't good and he scared my daughter with his big white eye, but he was fine-his eyes fine.
And I think they do actually ingest the bark off of applewood. I'd notice that. It probably tastes good so they eat it, but it isn't a treat, so don't worry-they can have applewood all day, everyday.