Examples of sedimentary rocks are limestones, sandstones, and shales. Igneous rocks result from the cooling of molten rock or magma to create rocks like granites, basalts, and rhyolites. Igneous and metamorphic rocks are not common in Kentucky but have been observed in glacial drift in northern Kentucky, and have been found as constituents in sandstones in eastern Kentucky and in very deep wells drilled throughout the State. The two main types of igneous rocks are intrusive, which formed when magma cooled slowly and hardened beneath the earth's surface, and extrusive, which formed when magma solidified after it reached the surface. Peridotites formed very deep in the earth near the mantle under high temperature and pressure, and were thrust explosively toward the surface where they intruded into the host rocks.
Peridotite consists in part of the minerals olivine and pyroxene.
The Elliott County peridotite is a special type of peridotite called kimberlite. Olivine is the most abundant mineral in the dikes and accounts for the greenish color of the rock. Most of the calcite in these igneous rocks is primary, meaning it is of magmatic origin. Kimberlites are famous for containing diamonds. The dikes in Elliott County are geologically similar to the diamond-bearing kimberlites in Kimberly, South Africa, and kimberlites in Arkansas, where diamonds have been mined extensively, but no diamonds have ever been authenticated from the Elliott County deposits.