Made of rock, digging through rock. Meaning that the balance is positive. Unless you were using iron pickaxes, in which case I probably ought to berate you for using a valuable tool on mining a less valuable material - since cobblestone is a renewable resource (renewable for as long as you have access to water, lave, and at least one bucket), using higher-tier pickaxes for generic mining is considered to be a waste of resources (though it does save some time, I know, that's the conundrum most Minecraft players face - "time or resources"). Right now the only way to semi-reliably find more than single pieces of iron is cave exploration. The same about coal. Strip-mining is good for reliably finding redstone and gold (because they are pretty much everywhere on the lowest levels, so you are guaranteed to get some), and for getting high quantities of cobblestone, but outside of that it's just a way to waste time maybe getting something else than the above things. Charcoal, Althea. Burn logs to get charcoal, and then use it instead of coal (they burn for the same time, and they both can be used to make torches). Sure, you'd have to make a small tree farm to do that, but for as long as you place a torch near every tree (so that they could grow at night as well as during the day), 9 or 12 trees is a very good start.