Concretions are held together by a mineral matrix (i.e. the “glue”) different from what holds the surrounding rock together. In this case the the matrix formed crystals with perfectly aligned facets; stand in the right place and it’s a mirror that reflects the sunlight to you, but move six inches and it’s just a red rock. There were hundreds of concretions on this stretch of beach, each glistening in the sun as I walked past.