Physical Geology
Spring 2005

MONTANA GEMSTONES

All pictures in this paper are from the web sites.

These pictures are being used for educational purposes only.

Montana Gemstones

Being I lived in Montana most of my life – I chose to write my research paper on the two main gemstones found in Montana. I grew up in southeastern Montana, right on the Yellowstone River with several gravel bars just below our place. I wish now I would have taken more time to learn about agates and done some rock hunting. I put all my time into the ranch and animals; I didn’t realize what treasures I was missing until my Geology 101 class.

Montana has two main gemstones, the Montana Agate and Sapphire. Both stones share the honor of being the States gemstone. Some less common stones are: Amethyst, Beryl, Garnet, Obsidian, Smokey Quartz and Tourmaline.

The agates are found along the Yellowstone River in southeastern Montana. This agate was given the name of Montana Moss Agate, and is usually a light creamy yellow to almost a clear color with moss embedded structures. The colors inside can vary from black to brownish red.(1)

Eastern Montana where I was raised was mainly a shallow inland ocean, with huge forest lining it’s shores with a swamp like atmosphere. Volcanoes erupting and spewing lava entombed parts of the forest in lava and ash. The Yellowstone area bulged, roared and flowed huge amounts of lava that destroyed thousands of acres of redwoods and sequoias for

hundreds of miles around. As the hot lava poured over the trees, it devoured most of the wood, but some of the ingredients of limbs and also their shape remained trapped in the cooling lava. This process continued for hundred’s of years creating layer upon layer of forests growing up and then being driven down under mega-tons of lava and ash. When things settled down and the rains came, the mineral laden silica water flowed into the pockets and cavities left behind by the dying trees and hot lava. As the flows of water slowly filled the air spaces with liquefied silica, Montana Agate was created. The gravel deposits that the agates are found in are from the Pleistocene age. Agate is a desired and varied form of “Chalcedony”, a variety of quartz. The agate is classified as a microcrystalline or cryptocystalline quartz and has the hardness of a 7.(2)

The first U.S. sapphires were found in gravel bars of the Missouri River in Clark and Lewis Counties of Montana around 1865. More discoveries followed: Dry Cottenwood Creek in Deer Lodge County around 1889, Rock Creek in Granite County around 1892 and Yogo Gulch in Judith Basin County around 1895. Other small amounts of sapphire are being recovered from Quartz Gulch in Granite County, Pole Creek in Madison County, Missouri River in Chouteau County, and Brown’s Gulch in Silver Bow County.(3)

Sapphires were first found in Montana in the late 1800’s by gold miners working the alluvial gravels of the Missouri River near Helena. The small multicolored sapphires were considered worthless (only gold was of value) and clogged the gold sluices which angered the miners. “Sapphire Collins” went to Helena in the 1860’s with a pocket full of pretty stones. He tried to convince local merchants and bankers that they had value with little success. But when Eastern and European fanciers learned of the sapphires in Montana – it didn’t take them long to invest in the sapphire mines throughout the state.(3)

Jack Hoover was the person to make one of the earliest discoveries of the Yogo sapphire. He was looking for gold in the gravels of Yogo Creek in 1895, and found the blue pebbles. (3)

The “Yogo” sapphire, a cornflower blue is unique among the world’s sapphires. The cornflower blue is natural and lack the color zoning prevalent in other sapphires. The clarity is uniformly high. All the colors of the sapphire are mined in Rock Creek Montana. The stones are high quality, but the quantity is limited. Most of the crystals found are too small to be cut. Finds over 2 carat are a rare find. The largest crystal ever found was in 1910. In the rough this crystal was 19 carats, by the time it was cut, it was an 8 carat stone.(4) Today, the Yogo sapphires are ranked among the finest natural sapphires in the world. The Yogo sapphire it totally untreated and is left in it’s natural state.

Color range of the Yogo Sapphire

The sapphire was formed deep in the earth; they were brought to the surface along a fault from volcanic action and are mined today at around 300 feet below the ground. It takes around fifty million years to produce a clean sapphire crystal with even distribution of color saturation. In Montana the sapphires were formed in a crustal metamorphic source with a low Gallium content.(5) Yogo sapphires are found in Madison Limestone that was formed around three hundred million years ago when Montana was covered by an inland sea. When the magmas cooled slowly over time, a dike was formed 3 to 8 feet thick and over five miles long. The dike is nearly vertical and lies seven thousand feet deep. The magma crystallized and atoms of oxygen combined with atoms of aluminum to form corundum, the mineral form of aluminum oxide.(6) The first Yogo sapphires were found by their original dike that was formed around 50 million years ago. They were brought to the surface by basalt magmas as xenocrysts.(7)