Concretion - Cabbage Form

This specimen is about 9" long and seen at a vendor from Madagascar at the Tucson Rock and Gem Show 2012. The label of this large pile of rocks says "Astimaliti" but I can find no reference online to the term. The best I can figure is that the specimens are a form of concretion that form around a central "seed", from sand, gypsum, mud or shale.

Done