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The background (matrix) of Montana agate ranges from nearly clear, through milky grey-blue to white. If the agate is composed of very fine layers, a small percentage of Montana agates may show rainbow iridescence (also known as iris agate). Some Montana agates, such as “limb casts”, appear to have originated from wood that became buried by ash and fossilized after the volcanic activity.
It's probably safe to say that all Montana agate has been transported many miles from its original location. The transport process has tumbled the agates along stream beds making the weak or cracked rocks fall apart. In some cases, cracked rocks will “heal” due to being exposed to the silica-rich waters of the Yellowstone River.
Sometimes, agates will have a little bubble of water trapped inside, creating liquid time capsules! This is called an enhydro. The water can be seen if you shine a light on the agate from behind, or heard if you shake it.
One of the most attractive features of Montana agate is the colored inclusions, which are made up of different elements (such as iron and manganese). These inclusions form when mineral-laden water is “wicked” into the rock through cracks or fractures and the mineral content of the water is deposited within the rock.
The highest-quality Montana agate is free from cracks or “checks”. Some of the most beautiful Montana agates contain dendrites, or tree-like plumes of mineral inclusions.
While some type of agate can be found in nearly every state, Montana agate is known by rock hounds around the country for the beauty of its designs. The agates are found over a broad area along the Yellowstone River valley and adjacent plateau areas from Emigrant, MT to the North Dakota border. They can be found along the entire length of the Yellowstone River, but the stretch between Forsyth and Sidney contains some of the most highly prized deposits. Different stretches of the river provide different concentrations, sizes and colors of agates. Agates with a reddish exterior are more common in the far eastern portions of the river, around Terry, Glendive and Sidney. Blue colors are more common from Forsyth to Miles City.
Rough Montana agate often resembles a potato, and is difficult to distinguish from other rocks. Montana agates may have developed a distinctive white crust, although the agates that are found away from the Yellowstone River are often more cracked than the rocks that are found along the river. Most people start collecting Montana agate by breaking likely-looking rocks with a hammer in order to learn to identify Montana agate. Experienced collectors try to avoid hammering the rocks in order to avoid ruining a potentially beautiful specimen.
Montana Agate Ring with a hand cut and polished cabochon by Beverly Jenkins