The Confusion Range in Millard County is the finding location for the so called “red horn corals”. But they dont belong to the same family like today´s horn corals - they are belonging to the rugosa corals.
The Rugosa are an extinct group of corals that were abundant in Middle Ordovician to Late Permian seas. Solitary rugosans are often referred to as "horn corals" because of their characteristic shape. However, some species of rugose corals could form large colonies.
Utah was during the Mississippian (~340 mya) almost completely covered by a shallow sea. Abundant horn coral fossils can be found in the Confusion Range in Millard County.
The cellular-structure of the petrified coral-fossiles is usually conserved by calcite. Specimens like that dont polish very well because of the softness of the mineral calcite. Is the structure of the cells preserved by reddish colored agate, the polishing will be very shiny.