Fossiliferous Siltstone, also known as Arabic Writing Stone, Mariam Stone, Elephant Skin Jasper, and many others. It is a sedimentary rock mostly made up of silt-sized particles. silt* has been compacted into rock by water, wind, or ice. Some may contain small fossils within the stone.

When raw stones are rubbed it will leave a streak of harmless reddish brown substance on the skin (see picture).
 
*Silt is loose granular particles.