This is an interesting example of a crystal-filled geode revealed in the host matrix rock. This rock came in a large batch of fossiliferous material from Ordovician strata in Shelby county Kentucky near Bardstown. This rock is known to contain numerous fossils from the Ordovician (approx. 440 to 485 mya) period, including crinoids, trilobites, and brachiopods. While splitting some of the material open, this geode was revealed in cross section. It was an unexpected and fun find. Elsewhere on this same slab of rock, there are some faint crinoid impressions. There is also something (likely a fossil) revealed on the backside of the slab - I cannot ID what it is, but something dark colored is exposed. There are also more faint impressions on the back.
Refer to the photos. The black centimeter cube is shown for scale and is not included. You are purchasing the specimen shown. It measures approx. 4.75 inches by 4.6 inches at it's widest points and weighs 337 grams. Your purchase will include an ID label.