Seemingly randomly, various stone blocks will exist that take significantly longer to mine and result in nothing dropped. Even when a diamond axe is enchanted with Unbreaking 3 and Efficiency 5, it will take as long as an obsidian block to mine it and result in no drops. The deeper one is (closer to Y=0) the more likely one is to find such a block. Also on Android version, both for as long as I can remember.