Lake Superior agate
Encyclopedia
The Lake Superior agate is a type of agate
stained by iron and found on the shores of Lake Superior
. Its wide distribution and iron-rich bands of color reflect the gemstone's geologic history in Minnesota. In 1969 the Lake Superior agate was designated by the Minnesota
Legislature as the official state gemstone.
The Lake Superior agate was selected because the agate reflects many aspects of Minnesota. It was formed during lava eruptions that occurred in Minnesota about a billion years ago. The stone's predominant red color comes from iron, a major Minnesota industrial mineral found extensively throughout the Iron Range
region. Finally, the Lake Superior agate can be found in many regions of Minnesota as it was distributed by glacial movement
across Minnesota 10,000 to 15,000 years ago.
. Magma upwelled into iron-rich lava flows throughout the Midcontinent Rift System
, including what is now the Minnesota Iron Range
region. These flows are now exposed along the north and south shores of Lake Superior. The tectonic forces that attempted to pull the continent apart, and which left behind the lava flows, also created the Superior trough, a depressed region that became the basin of Lake Superior.
The lava flows formed the conditions for creation of Lake Superior agates. As the lava solidified, water vapor and carbon dioxide trapped within the solidified flows formed a vesicular texture
(literally millions of small bubbles). Later, groundwater transported ferric iron, silica, and other dissolved minerals passed through the trapped gas vesicles. These quartz-rich groundwater solutions deposited concentric bands of fine-grained quartz
called chalcedony
, or embedded agates.
Over the next billion years, erosion exposed a number of the quartz-filled, banded vesicles—agates—were freed by running water and chemical disintegration of the lavas, since these vesicles were now harder than the lava rocks that contained them. The vast majority, however, remained lodged in the lava flows until the next major geologic event that changed them and Minnesota.
During the ensuing ice ages a lobe of glacial ice, the Superior lobe, moved into Minnesota through the agate-filled Superior trough. The glacier picked up surface agates and transported them south. Its crushing action and cycle of freezing and thawing at its base also freed many agates from within the lava flows and transported them, too. The advancing glacier acted like an enormous rock tumbler, abrading, fracturing, and rough-polishing the agates.
The gemstone comes in various sizes. The gas pockets in which the agates formed were primarily small, about 1 cm in diameter. A few Lake Superior agates have been found that are 22 cm in diameter with a mass exceeding 10 kilograms. Very large agates are extremely rare.
The most common type of Lake Superior agate is the fortification agate with its eye-catching banding patterns. Each band, when traced around an exposed pattern or "face," connects with itself like the walls of a fort, hence the name fortification agate.
A common subtype of the fortification agate is the parallel-banded, onyx-fortification or water-level agate. Perfectly straight, parallel bands occur over all or part of these stones. The straight bands were produced by puddles of quartz-rich solutions that crystallized inside the gas pocket under very low fluid pressure. The parallel nature of the bands also indicates the agate's position inside the lava flow.
Probably the most popular Lake Superior agate is also one of the rarest. The highly treasured eye agate has perfectly round bands or "eyes" dotting the surface of the stone.
Occasionally, collectors find a gemstone with an almost perfectly smooth natural surface. These rare agates are believed to have spent a long time tumbling back and forth in the waves along some long-vanished, wave-battered rocky beach. They are called, appropriately enough, "water-washed" agates.
can be used as a jewel when cut and polished. Only a fraction of the Lake Superior agate are of the quality needed for lapidary
. Three lapidary techniques are used on Lake Superior agates:
and the area around Thunder Bay
in Northwestern Ontario
, Canada
. Lake Superior agates have been found in gravel deposits along the Mississippi River basin.
The following characteristics are used to identify agates in the field.
The best time to find agates are when the sun is either rising or setting. Walking into the sunlight on a beach or gravel pathway will illuminate any agate on the surface. This works for almost any type of agate except black ones.
Agate
Agate is a microcrystalline variety of silica, chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks and can be common in certain metamorphic rocks.-Etymology...
stained by iron and found on the shores of Lake Superior
Lake Superior
Lake Superior is the largest of the five traditionally-demarcated Great Lakes of North America. It is bounded to the north by the Canadian province of Ontario and the U.S. state of Minnesota, and to the south by the U.S. states of Wisconsin and Michigan. It is the largest freshwater lake in the...
. Its wide distribution and iron-rich bands of color reflect the gemstone's geologic history in Minnesota. In 1969 the Lake Superior agate was designated by the Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
Legislature as the official state gemstone.
The Lake Superior agate was selected because the agate reflects many aspects of Minnesota. It was formed during lava eruptions that occurred in Minnesota about a billion years ago. The stone's predominant red color comes from iron, a major Minnesota industrial mineral found extensively throughout the Iron Range
Iron Range
The Iron Range is a region that makes up the northeastern section of Minnesota in the United States. "The Range", as it is known by locals, is a region with multiple distinct bands of iron ore...
region. Finally, the Lake Superior agate can be found in many regions of Minnesota as it was distributed by glacial movement
Wisconsin glaciation
The last glacial period was the most recent glacial period within the current ice age occurring during the last years of the Pleistocene, from approximately 110,000 to 10,000 years ago....
across Minnesota 10,000 to 15,000 years ago.
Geologic history
More than a billion years ago, the North American continent began to split apart along plate boundariesPlate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
. Magma upwelled into iron-rich lava flows throughout the Midcontinent Rift System
Midcontinent Rift System
The Midcontinent Rift System or Keweenawan Rift is a long geological rift in the center of the North American continent and south-central part of the North American plate. It formed when the continent's core, the North American craton, began to split apart during the Mesoproterozoic era of the...
, including what is now the Minnesota Iron Range
Iron Range
The Iron Range is a region that makes up the northeastern section of Minnesota in the United States. "The Range", as it is known by locals, is a region with multiple distinct bands of iron ore...
region. These flows are now exposed along the north and south shores of Lake Superior. The tectonic forces that attempted to pull the continent apart, and which left behind the lava flows, also created the Superior trough, a depressed region that became the basin of Lake Superior.
The lava flows formed the conditions for creation of Lake Superior agates. As the lava solidified, water vapor and carbon dioxide trapped within the solidified flows formed a vesicular texture
Vesicular texture
Vesicular texture is a volcanic rock texture characterised by a rock being pitted with many cavities at its surface and inside. The texture is often found in extrusive aphanitic, or glassy, igneous rock...
(literally millions of small bubbles). Later, groundwater transported ferric iron, silica, and other dissolved minerals passed through the trapped gas vesicles. These quartz-rich groundwater solutions deposited concentric bands of fine-grained quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
called chalcedony
Chalcedony
Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic...
, or embedded agates.
Over the next billion years, erosion exposed a number of the quartz-filled, banded vesicles—agates—were freed by running water and chemical disintegration of the lavas, since these vesicles were now harder than the lava rocks that contained them. The vast majority, however, remained lodged in the lava flows until the next major geologic event that changed them and Minnesota.
During the ensuing ice ages a lobe of glacial ice, the Superior lobe, moved into Minnesota through the agate-filled Superior trough. The glacier picked up surface agates and transported them south. Its crushing action and cycle of freezing and thawing at its base also freed many agates from within the lava flows and transported them, too. The advancing glacier acted like an enormous rock tumbler, abrading, fracturing, and rough-polishing the agates.
Description
The Lake Superior agate is noted for its rich red, orange, and yellow coloring. This color scheme is caused by the oxidation of iron. Iron leached from rocks provided the pigment that gives the gemstone its beautiful array of color. The concentration of iron and the amount of oxidation determine the color within or between an agate's bands. There can also be white, grey and red coloring on the rocks as well.The gemstone comes in various sizes. The gas pockets in which the agates formed were primarily small, about 1 cm in diameter. A few Lake Superior agates have been found that are 22 cm in diameter with a mass exceeding 10 kilograms. Very large agates are extremely rare.
The most common type of Lake Superior agate is the fortification agate with its eye-catching banding patterns. Each band, when traced around an exposed pattern or "face," connects with itself like the walls of a fort, hence the name fortification agate.
A common subtype of the fortification agate is the parallel-banded, onyx-fortification or water-level agate. Perfectly straight, parallel bands occur over all or part of these stones. The straight bands were produced by puddles of quartz-rich solutions that crystallized inside the gas pocket under very low fluid pressure. The parallel nature of the bands also indicates the agate's position inside the lava flow.
Probably the most popular Lake Superior agate is also one of the rarest. The highly treasured eye agate has perfectly round bands or "eyes" dotting the surface of the stone.
Occasionally, collectors find a gemstone with an almost perfectly smooth natural surface. These rare agates are believed to have spent a long time tumbling back and forth in the waves along some long-vanished, wave-battered rocky beach. They are called, appropriately enough, "water-washed" agates.
Cutting and polishing
A gemstoneGemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
can be used as a jewel when cut and polished. Only a fraction of the Lake Superior agate are of the quality needed for lapidary
Lapidary
A lapidary is an artist or artisan who forms stone, mineral, gemstones, and other suitably durable materials into decorative items such as engraved gems, including cameos, or cabochons, and faceted designs...
. Three lapidary techniques are used on Lake Superior agates:
- Tumbling—Small gemstones are rotated in drums with progressively finer polishing grit for several days until they are smooth and reflective.
- Saw-cut and polish—Stones up to 1/2 kg are cut with diamond saws into thin slabs, which then are cut into various shapes. One side of the shaped slab is polished producing fine jewelry pieces and collectible gems called cabochonCabochonA cabochon , from the Middle French caboche , is a gemstone which has been shaped and polished as opposed to faceted. The resulting form is usually a convex top with a flat bottom. Cutting en cabochon is usually applied to opaque gems, while faceting is usually applied to transparent stones...
s. (Note the value of large Lake superior agates, which weigh a few pounds or more, will lose most of their value if cut into slabs.) - Face polishing—Polishing a curved surface on a portion of the stone and leaving the major portion in its natural state is called face polishing.
Distribution of Lake Superior agate
One of the most appealing reasons for naming the Lake Superior agate as the Minnesota state gemstone is its general availability. Glacial activity spread agates throughout northeastern and central Minnesota, extreme northwestern Wisconsin, Northern Iowa, and Michigan's Upper Peninsula in the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and the area around Thunder Bay
Thunder Bay
-In Canada:Thunder Bay is the name of three places in the province of Ontario, Canada along Lake Superior:*Thunder Bay District, Ontario, a district in Northwestern Ontario*Thunder Bay, a city in Thunder Bay District*Thunder Bay, Unorganized, Ontario...
in Northwestern Ontario
Northwestern Ontario
Northwestern Ontario is the region within the Canadian province of Ontario which lies north and west of Lake Superior, and west of Hudson Bay and James Bay. It includes most of subarctic Ontario. Its western boundary is the Canadian province of Manitoba, which disputed Ontario's claim to the...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. Lake Superior agates have been found in gravel deposits along the Mississippi River basin.
Finding the gem
Typically the richly colored banding pattern is not well exposed and prospectors must look for other clues to the presence of agates.The following characteristics are used to identify agates in the field.
- Band planes along which the agate has broken are sometimes visible, giving the rock a peeled texture. It appears as though the bands were partially peeled off like a banana skin.
- Iron-oxide staining is found on nearly all agates to some degree, and generally covers much of the rock. Such staining can be many different colors, but the most common are shades of rust-red and yellow.
- Translucence is an optical feature produced by chalcedony quartz, the principal constituent of agates. The quartz allows light to penetrate, producing a glow. Sunny days are best for observing translucence.
- A glossy, waxy appearance, especially on a chipped or broken surface, is another clue.
- A pitted texture often covers the rock surface. The pits are the result of knobs or projections from an initial layer of softer mineral matter deposited on the wall of the cavity in which the agate formed. Later, when the quartz that formed the agate was deposited in the cavity, these projections left impressions on the exterior.
The best time to find agates are when the sun is either rising or setting. Walking into the sunlight on a beach or gravel pathway will illuminate any agate on the surface. This works for almost any type of agate except black ones.