chalkboards used to be made out of literal slate, this is where the expression "slated" referring to planned actions comes from "the halving is slated for 4/20, or near-abouts, on block 840000" (i think that's the block right?)

you wouldn't have the problem of fingernails on a slate, it's not rough in the same way as a chalk board

oof, sorry to make you think about that, gives me a cringe just thinking about it...