Skarn block
Skarn can be found at deeper depths in the crust and is often seen as the contact surface between limestone, marble and granite. Look for skarn in contacts between biomes! Skarn is one of the new iron and redstone (copper) ores, and replace the former blocks of iron ore and redstone ore.
Skarn is one of the bigger sources of metals in the mod (e.g. iron, rare earth elements, tungsten and copper).