Agates and sapphires are Montana’s two official gemstones. Although agates occur worldwide, dendritic agates from the Yellowstone Valley are highly valued for their unusual figurations. Often called moss agates, the interiors of these stones reveal startlingly realistic mini-landscapes when correctly cut and polished.
Agates are made when gases form bubbles within cooling igneous strata. These cavities are slowly filled with water carrying a silica solution tinted with mineral traces, usually iron. As the silica hardens, it forms regular bands of color of varying intensity. Successive layers of colored silica are laid down within the cavity. As the overlying rock is eroded, the nodes of agate are freed from their setting.
Moss agates are different from banded, or riband, agates because of the presence of plumelike formations within the stone. Small fractures allow the penetration of minute amounts of waterborne minerals into the silica node, which forms treelike or featherlike apparitions in the translucent stone as it hardens. Combined with bands of color within the agate, these formations make landscape images of trees and sunsets, or trees along a lakeside. The verisimilitude of moss agates can be uncanny.
These agates, often called picture agates, occur almost exclusively in the Yellowstone Valley between the mouth of the Bighorn River and the Missouri. Why this should be so is a matter of speculation. As the Bighorn, Tongue, and Powder Rivers drain a common area of Wyoming, some theories propose that the agates formed in volcanic ash and lava beds near the watersheds of these rivers and later washed downstream to the Yellowstone. This explanation seems rational, but no appropriate igneous formations have been found in Wyoming.
If you know what you are looking for, agates are not hard to find, but it takes a trained eye to spot them in the rough. Only if they are scuffed or broken do they reveal their translucent interior; otherwise they are a dirty yellow-white.
Agate hunting is a favorite pastime for many locals, and many shops carry baskets of cut agates or agate jewelry for those who are not willing to hunt for their own. The same locals and shopkeepers are usually willing to lend advice to novice agate hunters. The best agate-hunting seasons are early spring and midsummer, when snowpack runoff scours out the gravel beds. Inquire at the local chamber of commerce to find out if there are guided agate-hunting tours. Just be sure not to cross private property without permission.