Harder Deep Mining

To keep it simple: The lower you get the more indestructible blocks get.

Expanded upon:

Once you get to the y=0 area the game should transition from stone to a more durable block. With the latest snapshot you have implemented Grimstone. Grimstone should have a "hardness" somewhere between Stone and Obsidian and require an Iron Pickaxe to mine (Similar to how Obsidian needs diamond. I am not saying you can't mine it at all without one). Grimstone should NOT stretch all the way to the bottom of the world; instead, obsidian should replace both Stone and Grimstone somewhere around -32 and -16.

The entire idea should be to break mining up into stages and have deeper mining be a bigger challenge with the reward being more resources. With cave biomes equivalents to Grimstone and Obsidian should be added to further differentiate.

It would be cool to see enchantments/tools which specialize on these tougher blocks while neglecting weaker blocks.