A new Park County gem discovery: Tarryall fire agate

By Steven Wade Veatch Exceptional specimens of iridescent fire agate have recently been found in Park County in the USA, close to Tarryall Creek and near the Tarryall Reservoir. Fire agate is a variety of chalcedony (pronounced kal SED’ uh nee), a form of microcrystalline or cryptocrystalline (crystals too small to be seen without high magnification) quartz (SiO2). It contains inclusions of iron oxide (limonite) that produce an iridescent effect or ‘fire’. Chalcedony is generally formed near the surface of the Earth, where temperatures and pressures are low. The Tarryall fire agate has a botryoidal (grape-like) growth form. The agate is also layered: it contains thin layers of plate-like crystals of iron oxide in various planes. When light travels through these thin layers, the planes produce the iridescent colour play of red, gold and green. Fig. 1. Good fire agates are impressive in their rich and dramatic colour play. They form in cavities and cracks in the country rock from low temperature, silica-rich waters, in a way similar to how black opal forms. Lee Magginetti specimen. Photo date June, 2007, © by S. W. Veatch. The fire agate specimens were found as seams in granite near the Tarryall Creek. This  is a tributary of the South Platte River, approximately 25 miles (40km) long, in Park County, central Colorado. It drains a portion of north and central South Park, an intermontane grassland south-west of Denver. Tarryall Creek runs in several forks along the continental divide in the Pike National Forest and … Read More