Igneous extrusive layer

From Dwarf Fortress Wiki

Igneous extrusive stone is formed by lava solidification on the surface, as opposed to igneous intrusive stone which forms below the surface.

Igneous extrusive stone layers usually include Hematite, Gold, Copper, Tetrahedrite, and Native Aluminum.

Andesite, Basalt, Felsite, Obsidian, and Rhyolite layers are igneous extrusive.

Their name appears in a darker shade on the embark menu. They also indicate a good chance of magma being present in some form or another.