Cystiphyllum is a common fossil Rugose coral from the middle Devonian so it's no surprise to find them in the Jeffersonville Limestone near Louisville, KY. These are two specimens that caught my eye for their unusual look.
This first piece has had portions of it's Calcite skeleton replaced with white quartz.
The layering is very obvious in the specimen and they are due to regular growth of the coral.
Cystiphyllum gets it's name from the botryoidal, cyst like growth pattern that is found within the calice.
This next specimen is interesting because the quartz replacement is very irregular so it looks like a patchwork quilt.
This allows for "windows" where one can see the exterior wall and it's ornamentation while also seeing into the coral skeleton. I like how you can see the "cyst" shapes as concave shapes rather than the usual convex.