QUARTZ CRYSTALS (FIELD DIAMONDS) IN TENNESSEE

"Field diamond" or “Herkimer” type quartz crystals are found in many places throughout the world. In Tennessee they occur in two geologic environments and their overlying residual soils: firstly, scattered locations associated with breccias in the Lower Ordovician Knox Group dolomites, and secondly, within zones of folding and faulting in the Lower Middle Ordovician Sevier Shales. Their distribution may reflect geochemical zoning that also controls the distribution of zinc and barite deposits.

The greatest variety of crystal habits is found in the Knox breccia environment, where crystals often display morphology suggesting multiple stages of deposition under changing conditions. Breakage and regrowth observed in some specimens suggests tectonic activity. There is a consistent morphologic change from earlier more prismatic crystals to later bipyramidal overgrowths in the Knox crystals that points to a change in environmental conditions, perhaps an increase in temperature. Sevier Shale crystals tend to be simpler, usually bipyramidal in form. Crystals from both sources may also show evidence of later modification by processes of transport and erosion, and possibly by the action of moving groundwater. These crystals could eventually provide insight into the conditions surrounding the Alleghenian Orogeny, as well as factors that control open-space quartz deposition – whether silica takes the form of drusy quartz, agate, or field diamond type crystals.