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About Minerals

The Story of Montana Agate

It has always been a mystery how the peculiar little scenes got inside a rock as hard as agate. It is the claim of geologists that the spots were caused by infinitely minute seams of fissures in the softer parts of the rock being filled with metallic oxides when the world was young. These oxides made four different colors that form various combinations of color when blended together, or appear in single colors in each rock.

The red color is oxide of iron. The black is oxide of manganese. The green is oxide of copper. The blue is oxide of nickel. This theory has been elaborated by the help of high powered microscopes which show the tracings of little canals so close the naked eye could not detect it, but the oxides remained, staining the rocks in wonderful designs. The fern-like and branch effects of the trees, grass, and shrubbery, smaller canals from a common center, and in addition to these canals, the rock became flawed through shrinkage while passing through a period of evaporation which, according to scientists, has taken more than 3 million years to reduce the stone to the hardness of 7 points on the Mhos scale.

These canals and flaws have been perfectly healed by soft silicate formations of which the stone is a part, and the evaporation has caused the oxides to take on such forms as are seen on the window after a frosty night.

Technically, Montana agate is known as 'Dendritic' agate, and the moss spots are called Dendrites. It is the third hardest stone in the world, and is cut only with a diamond saw. There can never be two pieces alike even though cut from the same stone.

from Nevada County Gem & Mineral Society ROCK WRITINGS, via COBB-L-STONES 12/95 via Glacial Drifter 5/00