Jasper

Chalcedony variety colored by iron oxide

Jasper is an opaque type of silica that comes in a variety of colors, most commonly red, yellow, brown, green, and very rarely blue. It is made mostly of very fine-grained quartz or chalcedony, with other minerals contributing to its color and appearance. The common reddish color is due to iron inclusions.

From Wikipedia, under CC BY-SA. More on occurri.