What Is a Concretion and What Happened to the Fontainebleau Concretions?

I'd never heard of a concretion until a few years ago. Here's the dictionary definition of concretion: noun: concretion; plural noun: concretions a hard solid mass formed by the local accumulation of matter, especially within the body or within a mass of sediment. a mass of small concretions, each built up layer upon layer around some small nucleus" From this definition a concretion does not seem very appealing or interesting. Here, however, is one photo of the Fontainebleau Concretions in Richa...