Dunno if intended, but Stone Pickaxes aren't used for anything. You can craft them (to unlock a trigger tech of "Craft Stone Tools") - but that's it. I can't use them for anything. Shouldn't be there a recipe for Cobble in Manual Mine with them or something?

You’re right, thanks! The stone pickaxe’s only recipe (mining the iron vein in the Manual Mine) went away when the ore veins moved onto the map in 0.5.7, so it has no use right now. Tracked here: https://github.com/Rykon00/Gregtorio/issues/547. The plan is a cobblestone recipe with it at the Dig Site, between the wooden and the iron pickaxe.

Thanks for the report! Since the ore veins update the Stone Pickaxe had no use after the 10 that research Craft Stone Tools. In 0.5.9, Craft Stone Tools unlocks a Dig Site recipe “Mining Cobblestone With Stone Pickaxe”: 1 Stone Pickaxe and 4 manual labor give 64 stone in 10 s, between the Wooden Pickaxe’s recipe (8 manual labor) and the Iron Pickaxe’s (2 manual labor), so the pickaxes you craft for the research go straight into stone. Iron still comes from mining the map veins by hand at the start (about 4 s per ore, faster than a stone furnace smelts), and the Manual Mine takes over with Smelt Iron Ingots.