Montana agate has been a long time favorite of the lapidary enthusiast. Found in the alluvial gravels of the Yellowstone River in eastern Montana it has been collected and worked as a semiprecious stone for nearly a century. Noted for its clarity as well as its characteristic isolated black, and occasionally bright red dendrites, it makes excellent cabbing material. The majority of Montana agate pieces show a well-defined fortification pattern of black and brown as well as bright red/orange. There are also variations of filament and tube patterns and a small percentage display an “iris” band in which minute, regularly spaced layers act as a diffusion grating and, like a prism, break light into a rainbow of color. It is a relatively easy material to work with, being quite hard, and takes a brilliant polish.