The Morrison Formation dates to the Upper/Late Jurassic period. Approximately 146.8 to 153.3 million years ago, the area consisted of vast river systems and flood plains that provided excellent habitat for all manner of dinosaurs. Colorado, Wyoming and Utah are the states best known for their exposures of this sedimentary material, but outcrops can be found in other western states as well.
The majority of siliceous gem quality "coprolites" come from this formation and are generally attributed to herbivores. However, most lack readily identifiable inclusions such as undigested bone fragments or plant material. Unless this type of coprolite is found in direct association with fossilized animal remains or contains inclusions, it is impossible to determine if they were extruded by an animal or the earth itself. When the origin is unclear, they are referred to as dubiocoprolites.