Safety Enchantment

The Safety Enchantment would make it so that the tool or weapon would not affect certain materials or mobs.  The enchantment would not be available by placing a tool in the enchantment table, but could show up on a book.

So, once you've got a something with safety on it you go and try to pick/dig or attack something.  Until it's first used the item is 'unset'. You can set it by using it on a specific class of block or mob. Attack a baby animal with your sword or axe, and it gets attuned to baby animals. It will never damage baby animals again. Use a pick on a budding amethyst node... now it won't break budding amethyst nodes. Click an immature crop with a hoe and now your hoe won't harvest immature crops. There are probably other object/mob types that could be useful- maybe just one type of leaf for landscaping.  Putting it on your regular pick and having it not affect ores could be useful when you are insta-mining for collecting ore blocks, although just using silk touch could probably do that one as well. 

It might be nice on cooperative servers to be able to put it on weapons and set them so they don't hurt other players- particularly nice for sniping mobs when a friend is in melee or when you have sweeping edge on your sword.