Montana Agate

WHAT IS IT?

  • Agate is a cryptocrystalline (microscopic crystal) form of silica (silicon dioxide) usually found with metamorphic rock

  • Montana Agate has mineral colored inclusions

  • Red coloring results from iron oxide; Black from manganese oxide

  • The best grades of agates are free of cracks and have dendrites that have the appearance of tree-like plumes

  • Appearance: creamy light yellow to grey blue to clear with embedded brownish red to black moss

  • A small portion of agates have fine layering that produces rainbow iridescence. These are known as Iris Agates.

  • They are found in Pleistocene age gravel deposits (10,000 to 1.6 million years ago) along the Yellowstone Rivers and its tributaries

  • Hardness: —7

  • Agate is the birthstone for September

HISTORY

  • Formed from silica-rich volcanic ash originating in the Yellowstone National Park area

  • Some of the agates are "limb cast" that were formed when wood was buried beneath volcanic ash

  • Most of the agates travelled a long distance from their origin. Along the way collisions caused fracturing. Wicking of minerals (iron & manganese) into the fractures produced the colored inclusions characteristic of Montana Agate. In some of the agate, the fractures were "healed" (filled in) with silica by the mineral rich waters of the Yellowstone.

WHERE IS IT FOUND?

  • Found in Pleistocene age gravel deposits (10,000 to 1.6 million years ago) along the Yellowstone Rivers and its tributaries

  • Best collection sites are in a 50 mile stretch from Sidney, MT north to Glendive, MT in eastern part of the state

WHAT IS IT USED FOR?

  • Jewelry — especially cabochons

  • Collector specimens