Rhyolite is a mineral found at levels 1800-2000 in Dig Deep Mode just above bedrock. A Titanium Pickaxe or Sledge Hammer is required to mine it. It is the hardest and most valuable layer rock. Bedrock is the only harder block than rhyolite considering that nothing can break bedrock.
Rhyolite is completely blast resistant and acts as a very durable block to be used in structures. TNT, C4, boom arrows, and grenade launchers all fail to break even a single rhyolite block.