Composition ranges from material dominantly like that of the surrounding soil to nearly pure chemical substances entirely different from the surrounding material. Their form is apparently not governed by crystal forms based on examination at a magnification of 10X as is the case for crystals and clusters of crystals. It is impossible to be sure if some certain nodules and concretions formed where they are observed or were transported. Concretions are distinguished from nodules on the basis of internal organization. Concretions have crude internal symmetry organized around a point, a line, or a plane.
Nodules lack evident, orderly, internal organization.
A typical example of a concretion organized around a point is illustrated in figure 3-33 . The internal structure typically takes the form of concentric layers that are clearly visible to the naked eye. A coat or a very thin outer layer of an otherwise undifferentiated body does not indicate a concretion. Crystals are considered to have been formed in place. They may occur singly or in clusters.