Something that can't be shattered, cracked, or easily destroyed is unbreakable. If your new cellphone is truly unbreakable, it will hold up no matter how many times you drop it on the floor.
Use this adjective for anything that's physically durable, like the unbreakable glass used for car windshields or the unbreakable plastic bottles that ketchup comes in. It's impossible, or nearly impossible, to break these things. Things that are figuratively unbreakable can't be destroyed either, like the unbreakable bond between twins or the unbreakable secret code used by spies.