Cone-in-cone structures are strange features found as interbeds in fine-grained siliciclastic rocks and sometimes found in the outer layers of concretions. They consists of sets of conical structures stacked up within each other. Mineralogically, they are often composed of finely-crystalline fibrous calcite. Numerous hypotheses have been proposed over the years that try to explain cone-in-cone structures. None has emerged as the most popular or most likely (see Lugli et al., 2005 for a long list of proposed formation mechanisms). These structures remain a mystery.
Cone-in-cone structure (3.3 cm across), composed of finely fibrous calcite. This weathered sample is likely derived from an interbed in the Ohio Shale outcrop belt (Frasnian-Famennian, Upper Devonian) of central Ohio, USA
Specimen owned by Nicole Byrd.
For more info. on cone-in-cone structures, see (as examples):
Melichar & Shkovira (2001) - Case study of the cone-in-cone structure based on Czech and Crimean samples. GeoLines 13.
Lugli et al. (2005) - Silicified cone-in-cone structures from Erfoud (Morocco): a comparison with impact-generated shatter cones. in Impact tectonics. Impact Studies 6: 81-110.