Petoskey stones

Petoskey stone (4 cm in length) from Michigan.

A fossil commonly part of personal collections is a Petoskey stone, which is a fragment of a species of colonial rugose coral of Middle Devonian age (about 390 million years old) from lower Michigan. These fragments, which can be found along the shore of Lake Michigan, are commonly cut and polished (using lapidary techniques) into pebble- or small cobble-sized pieces with flat bottoms. Some are used in making designer jewelry.

The specimens show tightly packed, polygonal-shaped individual corals called corallites. The dark center of each corallite was the mouth area, surrounded by tentacles loaded with stinging cells used for catching prey. Radiating out from the center are distinct “lines” called septa, which were used for attachment of the soft parts of the coral. The name "Petoskey stone" comes from Ottawan Indian lore and means “rays of the rising sun.” It seems likely that this name was inspired by the presence of the septa. The Latin name for these Michigan specimens is Hexagonaria percarinata.

The Petoskey stones found in Michigan underwent transport and abrasion during the Pleistocene “Ice Age” by glaciers and running water, thus their colonial-coral structures have been worn down. When these specimens are polished by lapidary techniques, the colonial structures become even more worn.

For comparison, I include a photograph of a specimen of colonial-rugose coral that I collected from western Nevada. This specimen, which is of Mississippian age (about 350 million years old), is an  unworn example of Lithostrotionella jasperensis. The central area of each is corallite is raised and occupied by the columella, and the septa radiate from it.

Lithostrotionella jasperensis (4 cm length) from Nevada.

By the way, colonial-rugose corals went extinct at or near the end of the Permian Period (about 255 million years ago).

"Cantaloupe" concretions

A few years ago, while doing field work just west of Simi Valley, Ventura County, southern California, I collected some spherical concretions that are quite interesting. They are mostly five to six inches in diameter and have a distinctive pitted appearance that strongly resembles cantaloupes, thus I have dubbed them the name “cantaloupe concretions.” They are fun to collect, and other members of my field party fell victim of trying to find perfect ones (like the one shown below).  

A spherical concretion (approximately 11 cm = 4.5 inches) that looks like a cantaloupe.
 The black-and-white rectangles of the scale are in centimeters.

Some of the concretions are elliptical shape, as shown below.

An elliptical concretion (approximately 14.5 cm = 6 inches).

The concretions occur in sandstones in the nonmarine Sespe Formation and are approximately Oligocene (30 million years) in age. These concretions were first noted in 1924 by W. S. W. Kew, who did some of the earliest geologic work in the area. He also reported that they have a pitted appearance.

Concretions form around some nucleus (an impurity, possibly a shell or plant fragment) when groundwater flows through sands.  The sand subsequently becomes strongly cemented. Differences in the strength of cementation of the sand grains in these Sespe Formation concretions were made apparent by weathering of the surface, thereby producing the pitted appearance. Some of the concretions were broken open, but there was nothing organic nor unusual in the central (core) area.

In one of my earlier posts dealing with "Pseudofossils," in July, 2014, I briefly discussed what concretions are and illustrated a common shape.