There are four types of stone layers in Dwarf Fortress. Each has different types of ore and gems available. Layers are extremely important in site selection because it determines what metals there are to work with. Layers also help in determining where to begin exploratory mining depending on what is needed for the fortress.
Often includes coal, iron ore, and all but one of the flux stones, all the components for making steel. Never found in the same biome as igneous extrusive. Full to the brim with valuable and useful deposits. Only place to find bituminous coal and lignite, and iron ore in large clusters (magnetite). Good choice for exploratory mining.
Contains a small variety of ores, but tends to be rich in gems. Includes marble, the only non-sedimentary flux. If there are no flux sedimentary layers, metamorphic layers can be useful in the early stages, but the lack of weapon quality ore (besides copper and silver) make it a poor choice for full scale exploratory mining in the early game.
 Igneous extrusive
It is likely to include metal ores. It also indicates a good chance of magma being present in some form or another above the magma sea. Never found in the same biome as sedimentary. Igneous extrusive layers can contain veins of Hematite, an ore of iron.
 Igneous intrusive
Like extrusive, also often contains magma, but farther down typically. Often contains gold or copper ore.
 Other layers
When soil is present, it will always be the top layer or two, above any stone layers. Appears in brown on the embark menu. Useful for growing crops and storage, but provides no rock for building.
An aquifer is not a separate layer, but overlaps one or more layers of stone or soil. When viewing the layers on the embark menu, it is indicated by a line of blue tildes(~) next to the layer(s) involved. This layer of water-generating stone can be used as a water source, but also can be a potential barrier to all layers below it.
See Also: Stone