Random Hardness is a Minecraft mod that changes gameplay mining experience.

Features:

Stone now has 10 conditions. Different condition stones have different hardness and textures. All stones get certain states randomly during world generation.
Sulfur added. Sulfur ore generates in sandstone adjacent to air blocks.
Gunpowder can be made from two sulfur and one coal.
Creeper's drop is increased.
Villagers now trade gunpowder.

The purpose of the mod is to enhance the feeling of claustrophobia.

Primarily the mod increases break time of stone blocks. Now the player doesn’t want to break every stone block that annoys. As a result player’s tunnels are uneven, narrow and close. Also now you can’t dig a hole to the surface when you get lost in a cave. Actually, you can, but this is very tiresome, so you will rather find a way out of the cave.

The increased break time of stone blocks forces the use of TNT when developing a mine. New ways of gunpowder production, in turn, appeared.

Random Hardness can combine well with mods that change cave systems. For example, I liked to use Worley’s Caves mod with parameter noiseCutoffValue = -0.08. As well Burning Torches mod makes being underground uncomfortable which is also the purpose of Random Hardness mod.

Configurable parameters:

Break time of stone blocks with different degrees of cracks and some other solid blocks.
Cracks generation parameters.
Size of creeper’s drop.
Professions that can sell gunpowder.
Chance of sulfur ore vein generation.