Old book recommends bone charcoal for casehardening... after asking around, I learned this is because of its consistency, much less fluffy than most wood charcoals. Didn't harden my steel, but I WAS intrigued at this point.
Recently while hogging up some cherries, it allofasudden popped into my head that the cherry stones might be hard & dense enough to make a good charcoal which would be denser than oak charcoal, but maybe a little less dense than bone charcoal. Plus, if I stewed most of the soluble salts out of them, they'd be mostly carbon (guess you could use them for making art pencil sticks too)... unlike bones, which contain a lot of calcium, phosphate, traces of silicon, and many other things.
Provided a person could save up enough of them, I'd be inclined to think that cherry stones, apricot pits, peach pits, walnut shells, and pistachio shells, would all make good, dense charcoal. From what I understand, WW1 German gas masks were made with activated charcoal mfd from peach pits.
Guy Richards