calcium carbonate is the traditional primary component of blackboard chalk, and now that it's part of vanilla worldgen it seems like it should factor into a mod like this.

fwiw, calcite is pretty consistent in how it spawns, plus some other mods (quark being a notable example) make it show up in other places

Yeah, I thought about that earlier, but wasn't sure if this change would be good, as calcite is somewhat rare. But after a second thought it's probably wouldn't be so bad because of chalk spawning in chests.

Changed recipes to use Calcite in 1.1.0.