Ocracoke Island, Cape Hatteras National Seashore, North Carolina
The oyster and clam shell fragments dominating this “shell hash” are not their original color. These are secondary colors.
The yellow-brown stain is iron oxide, limonite. Shells acquire this stain when the iron minerals in the sand, ilmenite and magnetite, degrade and the mineral gets into the interstices of the shell. This happens when shells are exposed to oxygen on or near the surface of the beach.
The brown staining is “permanent,” as long as the shells spend occasional time in seawater, otherwise, the color fades or bleaches to gray. Numerous beaches along the southern US coast are littered with brown-stained shells.
Shells stain black with iron sulfide when they are buried in mud or rotting seaweed in the absence of oxygen. Often replenished beaches have black shells; this sand was either dredged or dug to replace sand lost to erosion.
Yohmom CC BY 3.0