Igneous intrusive layer
From Dwarf Fortress Wiki
Igneous intrusive stone is formed by lava solidification occurring below the surface, as opposed to igneous extrusive stone which forms above the surface of the earth.
Igneous intrusive stone layers may be made of granite, diorite, or gabbro, all of which typically contain gold and tetrahedrite.
Gabbro can also contain garnierite, the ore of nickel, kimberlite, which may contain diamonds, and olivine, which may contain native platinum. Granite can contain cassiterite, the ore of tin, bismuthinite, the ore of bismuth, and galena, the ore of lead. Galena will also give you a silver bar 50% of the time.
