Limestone, shale, and other sedimentary rocks commonly have scattered
throughout them masses of other rocks and minerals, such as limonite,
chert, and pyrite. These masses are called _concretions_. Concretions
may be round or oval, or they may have odd, irregular shapes. They—such
as some of the limonite concretions of east Texas—even may look like
gourds or sweet potatoes. Concretions generally are harder than the
surrounding rocks. Some are smaller than peas, but others are several
feet wide. (The word _nodule_ is used to describe small, rounded
concretions as well as other small, rounded mineral occurrences.)
It is believed that some concretions form at the same time as the rocks
in which they occur. Other concretions develop after the rocks
themselves have formed. These are deposited by underground water that
contains dissolved mineral matter. The water seeps through the rocks and
deposits mineral matter around an object in the rock, such as a fossil
or a grain of sand, to form a concretion.