A Mountain Ridge biome
Red Rock is a type of rock found only in Mountain Ridge biomes. When broken with a normal tool, they give you Red Cobblestone.
Sought mainly for its aesthetic look, when crafted into brick formation(2x2), these can give you 4 Red Bricks.
However, with IndustrialCraft 2 and GregTech, red rock can be macerated to produce Red Rock Dust. World Generation