MINERAL OF THE MONTH: July 2017 – Kona Dolomite

This rock is named after the Kona Hills, located in Marquette County, Michigan. This is an ancient formation of fossil stromatolite that is between 2.1 and 2.8 billion years old. Most dolomite found throughout the world is gray or white. Kona dolomite is quite colorful and is found nowhere else. Dolomite is a calcium-magnesium carbonate and fizzes in warm acid.  Dolomite is limestone with magnesium added. The Kona formation is dominantly dolomite with interstratified layers of shale, graywacke and quartzite. The dolomite beds are from a few inches to many feet thick, but even the purest beds contain thin cherty layers and other sedimentary clastic material.

The original organisms that created this fossil rock were some of the first life on earth. There was nothing to eat so these early cyanobacteria survived by using the energy of the sun via photosynthesis. Cyanobacteria were responsible for producing the first oxygen on earth as a byproduct of this photosynthetic process. Thus, if it were not for stramatolitic organisms, according to scientists, we would probably not be here.

The cyanobacteria grew in shallow water as flat mats to maximize exposure to the sun. The fossil rock formed as the cyanobacteria mats trapped, bound and, and cemented sedimentary grains. Over time, as the trapped sediments blocked the energy of the sun, the cyanobacteria would develop another mat on top of the mound. The mounds grow in various shapes including conical, stratiform layers, branching, and domal.

The earliest stromatolite mounds are believed to be 3.7 billion years old. They peaked around 1.25 billion years ago. Originally scientists believed these species of cyanobacteria were extinct, until live stromatolite mounds were discovered in Sharks Bay, Australia in 1958. Even though 99 percent of all species that have ever existed on earth are extinct – it is amazing that stromatolitic organisms were the first living things on earth – and they are still here. Living stromatolites mounds can be found in Australia, Chili, Brazil, Mexico, Belize, British Columbia and the Yukon (Canada), Minnesota (US), South Africa, and the Bahamas. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone.

Fossilized stromatolite rock can be found throughout the world. Kona dolomite is one example. Because of its tremendously old age, and due to trace minerals, Kona dolomite can be found in a variety of colors including pink, brown, yellow, tan, cream, red, and orange. The colors are often complex with mottling, banding and lacing. The black wavy lines in Kona are the remains of the microbial mats. The unique pink and red pigments are caused by the iron in the soil.

In the rough before polishing the rock does not look nearly as nice – but still cool.

Lapidary enthusiasts enjoy working with this rock because they make beautiful slabs used in crafting clocks, wind chimes, spheres, bookends, cabochons, and other items. Kona is also used for building stones and for making refractory bricks for furnace linings. Due to its vivid coloration it is also used for decorative purposes as crushed stone for borders and walkways.



  • Thanks to Bob and Peg Clark for their donations of Kona dolomite specimens and slabs to the Gitche Gumee Museum.

MINERAL OF THE MONTH: March 2017 – Chert

Chert is a microcrystalline or cryptocrystalline sedimentary rock made mostly of silicon dioxide (SiO2). It can form as nodules, concretions, and as layered deposits. Like other silica rocks and minerals, chert breaks with a conchoidal fracture, often producing very sharp edges. Native Americans took advantage of this fracture pattern and intentionally knapped chert to make arrowheads and other cutting tools and weapons. Since chert forms in sedimentary rock, it often can contain fossils as well as banded layers.

There are two main ways chert forms. In some cases, chert develops when microcrystals of silica grow in deposits of limestone or chalk. This occurs when dissolved silica is transported through sedimentary layers by groundwater. Large numbers of silicon dioxide microcrystals grow from the dissolved silica into irregularly-shaped nodules or concretions. If there is a lot of silica causing large number of nodules to form, the nodules can merge together to develop a contiguous layer of chert within the sedimentary rock. When chert develops from dissolved silica it is classified as a chemical sedimentary rock.

The other way chert forms is from biologic remains. Certain marine organisms contain silica in their exoskeletons or spicules, such as sponges, radiolarian, and diatoms. When these organisms die, their remains fall to the bottom of the oceans or shallow seas. The silica dissolves, recrystallizes, and develops into chert nodules or entire layers of chert.

Most chert is tan, cream color, or gray. When iron impurities are included within the nodules or layers, chert can also be red, green, or black. In some cases red chert, or chert with other colors, is classified as jasper. The term “flint” is used to describe varieties of chert that form in chalk formations, whereas chert usually forms in limestone formations. Some people make a distinction between “flint” and “chert” as a matter of quality – chert being lower quality than flint. Sometimes jasper is also considered a higher quality of chert.


MINERAL OF THE MONTH: June 2016 – Lake Superior Copper Replacement Agates

This web page update features the copper replacement agate. They can only be found in the Keweenaw Peninsula, located in Michigan’s Upper Peninsula. They can be found in the mine dumps near abandoned copper mines from the Kearsarge Lode. Most of these agates are small – less than an inch in diameter. They are extremely rare and difficult to find, usually requiring labor intensive work to free them from the basalt matrix rock. These agates are well sought after because of their rarity, their interesting patterns, and the vast array of other mineral inclusions. In addition to copper these agates often have other mineral inclusions including silver, calcite, malachite, tenorite, epidote, and pumpellyite, In most cases these small agates are fully husked, requiring them to be cut to expose their inner beauty.

The copper replacement agate shown below includes malachite, pumpellyite, and epidote inclusions.

The agate below has white chalcedony bands on the left and clear calcite on the right. Copper is in the outer shell, surrounding the specimen. The green is epidote and the red is a copper oxide.

The two photos below show copper replacement agate nodules in basalt matrix rock. In addition to copper there is also prehnite, epidote, and pumpellyite (dark green inclusions). These photos were taken by Dave Schuder.

The specimen shown below has basaltic matrix on the left and copper replacement on the right. The copper bands alternate with the chalcedony bands. This photo is from my agate book, Agates Inside Out.

CITATION: The first three photos shown above (used with permission) are from the Wayne W. Sukow collection http://www.sukowssuperiorminerals.com.

MINERAL OF THE MONTH: November 2015 – Slag

Throughout the past 17 years since re-opening the Gitche Gumee Museum (after it was closed by its founder 21 years previous), each summer many people bring pieces of slag in for identification hoping that the specimens are agate. Slag can be found on the beaches west of Munising, MI, as well as at several other places in Michigan in other areas where blast furnaces were used to purify ore. I can understand why people think these specimens are agate since oftentimes like agates; they have conchoidal fractures, structure or other patterns, and translucency. I have learned over the years how to let people down easily and educate them at the same time.

Originally there were 29 blast furnaces in the Upper Peninsula of Michigan that were used to melt down and purify iron ore.  Only two sites remain including the Bay Furnace in Christmas, and Fayette on the Garden Peninsula east of Escanaba.  Iron ore was first discovered in the U.P. in the 1840s.  Although the iron ore rock was up to 72 percent pure, it was necessary to remove the impurities and extract the iron.

Iron is purified from iron ore in a huge container called a blast furnace. Beginning in the 1840s in Michigan’s Upper Peninsula, iron ores such as hematite and magnetite were mined and then transported to blast furnaces.  To purify the ore, the rock was added to the blast furnace along with limestone and charcoal.  The mixture was heated to around 2282°F (1250°C) almost 300 degrees below iron’s melting point of 2786°F (1538°C).

In this reduction reaction, the charcoal was used to heat the mixture and add carbon to the chemical reaction.  The limestone served as a flux that helped to catalyze the desired reaction and chemically bind to and remove impurities, such as silica.  In this reaction, the iron oxide was reduced to iron, the carbon was oxidized to carbon dioxide, and the impurities were formed into glass-like slag, which was separated and removed. A picture of the Bay Furnace in its reconstructed condition is shown below.

CITES: https://en.wikipedia.org/wiki/Slag

MINERAL OF THE MONTH: June 2015 – Petoskey Stone (Michigan)

Okay, I will admit it. I didn’t realize until now that in this version of my webpage I have not yet featured the state stone for Michigan: the Petoskey stone. As I have changed webmasters over the last dozen years or more, the content of the webpage has also changed since I have started fresh with each new webmaster that had his or her own server. I have been with my current webmaster since May 2007 (thanks Michael!). So don’t you think it is about time that I include my state’s stone as mineral of the month?

A Petoskey stone is both a rock and a fossil. The rock is fossilized remains of a particular species of coral that lived during the Devonian period — rugose coral, Hexagonaria percarinata.

They are fragments of a coral reef that was originally deposited during the Devonian period (419–359 million years ago). At that time a shallow ocean covered what is now the lower peninsula of Michigan. Diagrams of the Michigan Basin and the rock that made up the geology of this basin are below.

The coral reefs that formed Petoskey stones are shown in the Devonian rock colored red in the two diagrams below. Specifically, it is found in the Gravel Point Formation of the Traverse Group.

When dry, the stone resembles ordinary limestone but when wet or polished the distinctive mottled pattern of the six-sided coral fossils emerges.

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). Most limestone is composed of skeletal fragments of marine organisms such as the coral that makes up Petoskey stone. Limestone makes up about ten percent of the total volume of all sedimentary rocks in the Earth’s crust.

These state stones can be found because they were plucked from Michigan’s bedrock by glaciers that last retreated from the area around 10,000 years ago. Erosional forces ground off the rough edges of these pebbles and deposited them primarily in the northwestern (and some in the northeastern) portion of Michigan’s lower peninsula. In these areas, complete fossilized coral colony heads can be found in the source rocks for the Petoskey stones. In 1965, it was named the state stone of Michigan.



MINERAL OF THE MONTH: February 2015 – Condor Agate (Argentina)

The condor agate was discovered and named by Luis de los Santos in 1993 in the Andes Patagonia Mountains near San Rafael, in Mendoza Province, Argentina. This agate exhibits vibrantly colorful bands and patterns, and has become a popular stone among collectors and jewelry designers. Close up photos of the condor agate shown above follow.

Luis had collected rocks as a child. This interest was rekindled when he saw a carved stone egg made from rhodochrosite, a local Argentinian mined mineral. On one of Luis ‘s visits to the Catamarca Province, he happened to talk with and old friend of Dr. Franz Mansfeld, a German geologist noted for his expertise regarding the Catamarca rhodochosite deposit. This friend told Luis that the geologist had also seen an agate in the Northern region of Patagonia. Luis went in search of these agates, so he made repeated visits to the area.

Rather than randomly hiking the mountainous region, Luis saw a broken piece of banded agate being used as a doorstop by one of the locals. He finally tracked down the agate deposit in an expansive area consisting of low hills located a considerable distance from the nearest road. The only way to get to the sight was on horse-back. The impressive agates were scattered over the surface of rock outcrops made of volcanic rhyolite and andesite. As it turns out, the agate nodules had naturally formed in hollow pockets within this igneous rock. The specimens spread over the surface had weathered out of the matrix rock. Realizing he had made an important find, Luis collected what he could and headed to the United States to sell the specimens. Theyt sold instantly and have been selling ever since. After several years of collecting, cutting, and polishing, he did not continue the business. However, his former wife, Ana de la Santos, picked up where Luis left off and has intensified the business since 2008. Ana now travels to Argentina twice a year and hires a crew to help her mine. Today, the surface specimens are long since gone, so mining requires digging shallow pits to find the nodules.


MINERAL OF THE MONTH: September 2014 – Black Corundum

Corundum is a crystalline form of aluminum oxide (Al2O3) with traces of iron, titanium and chromium. It can be found as a component in rocks as well as in pure crystal form. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems — Ruby and Sapphire. which are scientifically the same mineral but just different colors. Ruby is red and Sapphire is the variety that encompasses all other colors, although the most popular and valued color of Sapphire is blue.

Because of corundum’s hardness (pure corundum is has a hardness of 9.0 Mohs), it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large machines. Corundum is the third hardest natural mineral known to science. The hardest mineral, diamond is four times harder than corundum. The second hardest is Moissanite (Silicon Carbide) at 9.25. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.

Corundum can be found in stream and beach sands because of its hardness and resistance to weathering. The largest documented single natural crystal of corundum ever found measures about 65×40×40 cm (26×16×16 in), and weighs 152 kg (335 lb).

Corundum can not only be artificially synthesized, but even natural gem stones are often heat treated to enhance their color.

The specimen featured in the picture above was acquired from Pierre Trudel, who recently visited the Gitche Gumee Museum from Quebec, Canada. The smaller specimen shown in close up detail below was donated by Pierre.

Below are a couple of close up photos that I took of this specimen of black corundum.

Metaphysical Properties

According to the web page http://www.mineralminers.com/html/corminfo.htm#metaphysical-corundum-information , “Corundum is said to enhance one’s creativity, self-confidence, awareness, intuition and ambition. It is also said to help release negative emotions in constructive ways while giving give inner peace.”


MINERAL OF THE MONTH: April 2014 – Tiger’s Eye

Tiger’s eye (also called Tigers eye or Tiger eye) is an unusual quartz rock that forms when silica replaces a fibrous form of blue asbestos (crocidolite). Rocks that form under this replacement process are called pseudomorphs. It comes in various colors, but is naturally golden in color. Red tigers eye stones are manipulated with heat treatment. Darker tigers eye has been altered using nitric acid.

Here is a picture of natural, untreated, unpolished tigers eye.

Tigers eye is especially known for its chatoyancy. Chatoyant minerals are comprised of tubes or fibers that form in a perpendicular arrangement. When light hits these rocks, a luminous streak of light is reflected perpendicular to the direction of the fibers, imitating the look of a feline’s eye.

Notable sources of tiger’s eye include Australia, Burma, India, Namibia, South Africa, United States, Brazil, Canada, China, Korea, and Spain.

Tigers eye is thought to be one of the most protective stones. It can also promote balance, clear emotions, promote a positive attitude, and encourage calmness and peaceful disposition.


MINERAL OF THE MONTH: November 2013 – Skip-an-Atom Agate

Although controversial, some people classify Skip-an-Atom Agates as s a sub-variety of Lake Superior Agate. These rocks are usually opaque with patterns of large quartz crystals with evenly distributed tiny air spaces that sometimes manifest with a lilac color cast.

These Lake Superior nodules are usually found in and around Duluth, MN – and are quite rare. In cases when these nodules were moved by glaciers or icebergs, they can also be found elsewhere including on the beaches of Grand Marais.

During the last few years rockhounds around the Mid-west have discussed these silica nodules. It seems that there is not agreement on several fronts. People do not agree on whether these specimens are in fact agate. They do not agree on exactly what constitutes a Skip-an-Atom Agate or how you can determine whether you have found one. Most agree that these specimens have a quartz ball center with a weird macro-quartz pattern. It does seem that the crystalline structure within these specimens was chemically or thermally altered causing the silica to recrystallize.

Apparently these agates were named by an elderly rockhound who thought the quartz looked so unusual that it must have “skipped an atom” in its formation. Thus, this is a descriptive term and does not have any scientific validity.

Many Skip-an-Atom agates are characterized by their notched and indented quartz crystals. The pattern of quartz crystals in Skip-an-Atom agates have been described as crenelated – like the indented/notched battlement top of the walls on an old fort (as shown below).

There are many theories about how these specimens formed. Some feel that these rocks formed due to the heat and pressure of a meteoric impact since the crystalline pattern resembles the shocked quartz found at impact sites (sample shown below).

Another hypothesis is that these “agates” are metamorphosed silica-rich schist called sericite. Schist is a type of metamorphic rock that is chiefly comprised of flaky minerals and exhibits a plated structure. A quartz sericite is shown below.

Typically schists contain significantly lower levels of quartz, but quartz-sericite schist is a variety that does contain more silica. As its name implies, this type of schist also contains significant amounts of sericite, which is a fine-grained type of either muscovite or paragonite, both of which are relatively abundant silicate minerals. Found in various locations, one of the best known examples of quartz-sericite schist occurs in Canada, where it is a source of economically important gold deposits.

A more plausible explanation is that contact metamorphism formed these rocks when hot molten lava intruded preexisting agate bearing rock causing the transformation of the silica in the agate nodules that were still resident in the vesicle pockets.

Here are some more photos of a Skip-an-Agate found by local resident, Kim, in Grand Marais, Michigan.


MINERAL OF THE MONTH: August 2013 – Unakite

This rock was first discovered in the United States in the Unakas mountains of North Carolina. Unakite is an altered form of granite composed of pink orthoclase feldspar, green epidote, and quartz. The green epidote can vary in its shade of green, as can the pink feldspar. Usually the colors combine to create a mottled appearance. In some cases, when the colors are especially pronounced, unakite is considered a semiprecious stone and can take a good polish. Since it does polish well, unakite is often used to make jewelry and other trinkets.

Unakite can be found as pebbles and cobbles from glacial drift in the beach rock on the shores of Lake Superior, and sometimes Lake Michigan. It also has been found in Virginia, South Africa, Sierra Leone, Brazil, Zimbabwe, Switzerland, and China.

Unakite derives its name from the Greek epidosis, meaning “growing together”. This stone can be used to draw off negative energy, therefore lifting your spirits. It is also used to help you to see and appreciate the beauty of life and to help you overcome challenges and negative experiences – especially those that drain your energy and try to hold you back.