They are concentrically laminated pyrite grains of up to 2 mm diameter. Comparable occurrences are found in other sedimentary basins. How they form is not known, but a popular idea is that they formed as a replacement of precursor iron or carbonate ooids. The examples from the Chattanooga Shale shown on this page, however, suggest that they might be of primary origin. I would be most interested to hear about it.
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Click on the image to get a larger, high resolution picture. SEM images from broken pyrite ooid. Bottom photograph shows concentric nature of pyrite cortexes. This observation suggests that pyrite ooids in core formed essentially at the seafloor, and were pyritic prior to incorporation into ooid. Scale bar 0.1 mm long .