The Montana Moss Agates are each one of a kind. Iron oxides tint the agates to red and manganese spread into layers making the famous black dendrite tree and flower patterns. Internal botryoidal patterns often show as bubbly cloud like formations on highly polished specimens.
The Montana Moss Agates formed over 65 mission years ago as volcanoes began erupting along the eastern front of what is now the Rocky Mountains. For millions of years massive lava flows buried the land including entire forests. Most agates from around the world, including the Montana Moss Agates, were formed within cavities and gas pockets within the lava. Most of these cavities were caused by trapped steam and gasses trying to escape through the lava as the lava cooled. Mineral water with silica (SiO2) flowed in and out of these cavities leaving behind layer after layer of agate.
Between 40 and 50 million years ago the central Montana Mountains were forming. The lifting and shifting movement of the continental plate caused the break up of the original lava beds containing the agate nodules, limb casts and petrified wood. For tens of millions of years the harder agates eroded out of the softer basalt. During the last 2 million years glaciers and rivers carried the agates east along a wide basin, where the Yellowstone River now flows.
Cooke City Rocks has a good selection of Montana Moss agates.