Montana: The "Treasure State"
One of Montana's popular nicknames is "The Treasure State." This nickname was inspired by the many mineral resources that are found in Montana. Gold has been produced from the sediments of many streams. Gold, silver, and numerous other metals have been produced from hard rock deposits.
Two gemstones, sapphire and agate, have been so popular in Montana that the state legislature designated them as "official state gemstones" in 1969.
Sapphires were discovered in Montana in the 1860s. Since then millions of carats of "Montana Sapphires" have been produced, and at least one million cut stones have entered the jewelry trade. The people of Montana and visitors to the state have become enthusiastic patrons of a small sapphire-mining industry.
Beautiful agates have been found at many locations in southeastern Montana. They have been persistently popular with local rockhounds for over 100 years. Montana Moss Agate and Dryhead Agate have a popularity that extends far beyond the borders of the state.
Numerous other gem materials have been produced in Montana. These include: jasper, garnet, varieties of quartz, rhodonite, serpentine, staurolite, topaz, tourmaline, and wonderstone.
A few diamonds, diamond indicator minerals, and kimberlites have been found in the state. These signs indicate that a deposit worthy of mining might be found. If diamond hunters are fortunate, Montana could have the only commercial diamond mine in the United States.
Montana Sapphires: Alluvial Deposits
In the early 1860s, very few people lived in the Montana Territory. That began to change with the discovery of gold at several locations. Within a few years the population was growing rapidly, and the search for gold motivated people to carefully examine the sediments of every stream in the state.
Many of these prospectors noticed the colored sand grains and pebbles that lingered in their gold pans and sluices after the more common sediment particles were washed away. Most prospectors were totally focused on the search for gold and discarded them. Many who noticed the colorful grains did not know that they were corundum, the mineral of ruby and sapphire. Their high specific gravity (3.9 to 4.1) caused them to be concentrated in the same sediments as placer gold.
Eventually a few people realized that these colorful stones were sapphires, but they were not motivated to collect them. Why? Most of these stones did not have the blue color of highly marketable sapphires. Also, assessing the quality of the stones required expertise in gemology. The miners did not want to collect pebbles, only to find out that their big lot of "finds" contained just a few specimens of value. Finally, in the mid-1800s very few people in the United States had the skill and equipment to cut a stone as hard as a sapphire - with a hardness of 9 on the Mohs Hardness Scale. Thus, there was no apparent market.
Sapphires were abundant in some parts of Dry Cottonwood Creek, Rock Creek, and some sand bars in the Missouri River. Unfortunately they were not quite commercial and attracted very little interest.
Then, in the 1980s gem treaters in Thailand discovered how to heat white and yellow sapphires to a commercial blue color. Heat treatment of sapphires became such a common practice that most sapphires entering the gem market today had their color improved by heating.
These advancements in gem treatments converted many of Montana's previously worthless sapphires into gems of commercial quality. These were quickly mined during the 1990s, and several million carats of Montana sapphires entered the gem marketplace. Sapphire mining continues in Montana today, and much of the mining is done by hobbyist miners.
Montana Sapphires: Hard Rock Deposits
In 1879, prospectors found small amounts of gold in Yogo Creek, a small stream in central Montana. They didn't find much gold, but many panners noticed bright blue pebbles in the stream and an unusual rock formation a few feet in width cutting a limestone unit in the lower part of the drainage.
These prospectors didn't realize that the blue pebbles had the same hue and saturation as the most highly desirable blue sapphires. Perhaps they heard that the sapphires found in other parts of the state were not worth collecting. Nevertheless, when they left Yogo Creek they walked away from what would later be described as the greatest sapphire deposit in the western hemisphere.
In 1894, a property owner sent a small box of blue pebbles panned from Yogo Creek to a gold assayer, who didn't know what to do with them, so he forwarded them on to Tiffany's in New York City. At that time Tiffany's was considered to be the scientific authority for gemstones in the United States. George Kunz, the chief gemologist at Tiffany's, called Yogo Sapphires "the finest precious gemstones ever found in the United States."
The unusual rock formation that cut the limestone unit was later determined to be a vertically dipping dike of sapphire-bearing igneous rock. At the surface it weathered into a soft mud that was favored by gophers, whose burrows made the dike easy to trace across the landscape's surface for a distance of a few miles.
The sapphires of Yogo Gulch have now been famous for over 100 years. Millions have been produced, and although the deposits have been heavily prospected, production continues today at a very low rate.
Yogo Sapphires have been produced from both rock and alluvial deposits and in a wide range of colors which include blues, blue-greens, greens, pinks, pale reds, purples, yellows, and oranges. Much of the production is from dig-for-fee operations. Some gem-grade garnets are also produced from alluvial deposits, but most are industrial quality.
In addition to the igneous dike at Yogo, Montana sapphires are hosted in schist and gneiss in several parts of the state. These rock units are rarely mined because they are very hard and costly to excavate. In addition, the work of excavation damages many of the sapphires.
Montana Moss Agate
Montana Moss Agate is a transparent to translucent chalcedony found in the Yellowstone River Basin of southeastern Montana. It is often clear but can have a translucent white, gray, yellowish, or reddish brown base color. Similar material is found in northern Wyoming and it is often called "Montana Moss Agate."
Montana Moss receives its name from its black dendritic to mossy to geometric-shaped inclusions. This distinctive appearance is what makes Montana Moss Agate recognizable and popular. The black inclusions are thought to be a manganese oxide. The reddish brown color of some specimens is thought to be caused by small amounts of iron oxide in the agate.
These agate nodules formed in cavities and voids of the igneous bedrock and ashfalls that occur in this area. The nodules often have concentric banding with occasional drusy centers. They are much more resistant to weathering than the bedrock, and that concentrates them in soils and stream gravels.
Montana Moss is an abundant agate. It occurs over a broad geographic area instead of at a single deposit. This wide distribution was created by regional magmatic and hydrothermal activity of the Yellowstone Region.
Dryhead Agate is one of the most beautiful agates found anywhere. It is found in an area of southeastern Montana bounded by the Big Horn Mountains, the Pryor Mountains, and the Big Horn River. In this area, oval-shaped agate nodules are found as float in the soil and in stream sediments.
The quality of the nodules varies. Most are only a few inches across. Some of the best ones have a thick chocolate-brown rind and a fortification agate interior. Bands of agate within the fortifications can be white, yellow, orange, pink, red, or brown in color. The centers can be solid agate, drusy quartz, or hollow. Lower quality specimens can be vuggy or have calcite mixed with the agate. Those with thick areas of colorful banded agate produce the best cabochons. Before cutting one into slabs, you should know that many collectors of natural agate will pay a spectacular price for them. Hundreds of dollars are often paid for half of a sawn and polished Dryhead Agate that is only a few inches across.
Areas where the best agates are known to occur were mined by individuals during the 1950s, 1960s, and 1970s. In the 1980s, some areas were open to collectors on a fee mining basis. Today, very little new Dryhead Agate enters the lapidary market. What is currently being sold is mostly from old stocks.
Diamonds in Montana?
In 1990 a fourteen-carat diamond, one of the most famous discovered in North America, was found in Montana. It was found by Darlene Dennis, who was out for a walk on a rural road near the community of Craig in Lewis and Clark County. It was named the "Lewis and Clark Diamond" after the county in which it was found.
The stone was a rounded octahedral crystal without sharp points and edges. She sold it to a New York gallery for $80,000. The stone attracted a lot of attention to the area where it was found, but to date, no one has identified a viable source of diamonds in the area.
Small diamonds, diatremes, kimberlites, and diamond indicator minerals have been found at many locations in Montana. None of them have led to the development of a mine or a large-scale exploration dig.
Author: Hobart M. King, Ph.D.
|Ruby and Sapphire|
|Pictures of Opal|
|United States Gemstones|
More From Geology.com: