The ores you can find Beneath are mostly similar to what you can find on the surface, but with a few changes. Consult the Ores in TFG category for exact details.
The most common rock type found is Migmatite. This deep metamorphic rock contains a wide variety of ores you'd normally find in other rock types on the surface.
In wetter areas, you may find Travertine. It is a sedimentary rock that can contain the same kind of ores as Migmatite.
Around the deepest level of the Beneath, the different rock types gradually all turn into Pyroxenite. This ultramafic igneous rock is extremely rich in ores, but this depth also comes with extreme heat, so be prepared!
Anthracite is a powerful fuel. It can be found in Migmatite in the upper half of the Beneath.
Sylvite is a fertiliser that can be found in Pyroxenite in the lower half of the Beneath.