From DwarfFortressWiki

There are four layers of stone in Dwarf Fortress. Each has different types of of ore and gems available.

Contents 1 Sedimentary 2 Metamorphic 3 Igneous Extrusive 4 Igneous Intrusive 5 Other Layers 5.1 Soil 5.2 Magma 5.3 Aquifer

[edit] Sedimentary

Appears in a lighter shade on the embark menu. Often includes coal, iron ore, and flux, all the components for making steel. Never found in the same biome as igneous extrusive.

[edit] Metamorphic

Contains a small variety of ores, but tends to be rich in gems. Includes marble, which makes an easy to find flux.

[edit] Igneous Extrusive

Appears in a darker shade on the embark menu. It is likely to include obsidian and metal ores. It also indicates a good chance of magma being present in some form or another. Never found in the same biome as sedimentary.

[edit] Igneous Intrusive

Often contains gold or copper ore.

[edit] Other Layers

[edit] Soil

Appears in brown on the embark menu. Useful for growing crops, but provides no rock for building.

[edit] Magma

Not visible on the embark menu, unless you build directly on a volcano. Both useful and dangerous.

[edit] Aquifer

Appears as wavy blue lines on the embark menu. This layer of wet stone can be used as a water source, but acts as a barrier to layers below it.