This article discusses a few of the more prolific localities in southeastern Utah that produce colorful microcrystalline silica, which takes a good polish and is suitable for lapidary purposes (fig. 4). Microcrystalline silica includes banded agates and other forms of chalcedony, jasper, or, more commonly, combinations of the two. The difference between chalcedony—this includes all agate varieties—and jasper is their respective microcrystalline textures. Chalcedony (again, all agates) is mainly microfibrous, whereas jasper is mainly microgranular. Agates can be either the typical banded varieties or exhibit some other visually apparent textures such as those found in moss agate or plume agate, or they may be silica that exhibits a high degree of translucency, such as carnelian. Jasper is less translucent and can be almost opaque due to inclusions of iron or manganese oxides, clays, carbonates, or other materials. Combinations of chalcedony and jasper are extremely common, especially when small areas of fortification agate are mixed with masses of jasper material. A common term for this occurrence is jasper-agate.
Richard D. Dayvault, a geoscientist, is a consulting editor of Rocks & Minerals. His most recent article for the magazine, “Conifer Cones from the Jurassic and Cretaceous Rocks of Eastern Utah,” was coauthored with H. Steven Hatch and appeared in the September/October 2007 issue (pages 383–396).