Ores and Minerals
Beneath the overworld, you will still find ore veins to exploit. It is important to note that Netherrack does not collapse, like raw stone would. However, Blackstone, Nether Bricks, Cobblerack and miscellaneous other blocks do. Prospecting will still work Beneath the overworld.
Nether Gold
Nether Gold replaces netherrack, and is common. Beneath lava, there is more gold, and richer.
Cursecoal
Cursecoal is a powerful fuel. It replaces netherrack above level 80.
Sylvite
Sylvite replaces basalt at any level.
Nether Quartz
Nether quartz replaces netherrack at all levels, and is common.