Concretions are hard structures that form among sedimentary deposits early in the process of rock formation. They form when minerals concentrate and harden in one area before the surrounding areas. Chert and limestone concretions form in concentric layers as minerals build up around a small nucleus such as a pebble, leaf, or bone. They grow over a long period to form disks and other shapes of up to several meters in diameter. Irregular sandstone concretions form within sand sediment when minerals are deposited unevenly, when they concentrate in air pockets, or when they aggregate in crystal-like bodies. Despite some resemblance to fossils, concretions are not fossils.
Disk-shaped chert and limestone concretions are common on Highway 5 between Azraq and Jafr. They are naturally exposed by erosion or excavated for commercial use. Also found on exposed hillsides of western Jordan, including south of Petra at the GPS coordinates: 30.1380°, 35.3640°. Irregular sandstone concretions are found in Wadi Dana, Wadi Rum, and Petra.