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.


MINERAL OF THE MONTH: March 2013 – Jacobsville Sandstone

Jacobsville Sandstone is a red sandstone that has light-colored streaks and spots caused by chemical leaching across the bedding planes. This beautiful rock is found along the Lake Superior shoreline in northern Upper Michigan and Ontario. Scientists also think that Jacobsville lies under much of Lake Superior. There are also other sandstones in Wisconsin and Minnesota that may or may not be the same formation (Freda sandstone, Bayfield Group sandstone). Jacobsville Sandstone is composed of: 27.4% nonundulatory quartz, 27.0% undulatory quartz, 23.0% potassium feldspar, and 12.3% silicic volcanic clasts.

A hundred years ago this rock was mined and used as a building material. It was highly desired for its durability and aesthetics. Over 30 different quarries in the Upper Peninsula extracted Jacobsville sandstone between 1870 and 1915.

Below is a table showing the sandstone sales from 1899 to 1913.

Year Total Value
1899 $178,038
1900 $132,650
1901 $174,428
1902 $188,073
1903 $121,350
1904 $74,868
1905 $123,123
1906 $65,395
1907 $53,003
1908 $39,103
1909 $36,084
1910 $31,233
1911 $12,985
1912 $16,438
1913 $19,224

There is some confusion about this rock since it has had several names including redstone, brownstone, Lake Superior Sandstone, and Eastern Sandstone. In 1907, however, the sedimentary formation was given its current classification and the name Jacobsville, in honor of the town in which some of the quarries were located. There is also disagreement about the age of the sandstone. Most believe that it formed around a billion years ago, but others feel that it is half that age. Since the sandstone is entirely devoid of fossils, the older age classification is more likely.

Today, the best place to see this rock is near Au Sable Point, located around eight miles west of Grand Marais.


MINERAL OF THE MONTH: September 2012 – Chlorastrolite Greenstone

The mineral of the month is Chlorastrolite, also known as Isle Royale greenstone. This mineral was designated as the official state gem by Michigan’s 76th Legislature (Act 56, PA 1972). This legislation was introduced by Representative Russell Hellman of Dollar Bay. This mineral is a green variety of the mineral pumpellyite, and has a pattern of star-like crystals which forms a turtle-shell pattern. Greenstones are found in the waters and on the shores of Isle Royale, where it is illegal to collect due to the island’s national park status. It can also be found in the Michigan Copper ranges in the western Upper Peninsula. Many of the old tailings left over from the mining era have greenstones in them. They appear as dark green, small round or almond shape nodules in the basaltic rock. In some cases these nodules have weathered out of the volcanic rock and can be found along the shoreline. Unfortunately, only a small percentage of the green nodules are actual greenstones, as most are chloride, prehnite or some other mix of minerals. And of course, of all the greenstones found, perhaps only a few out of every hundred are gem quality.

It is difficult to identify an unpolished pebble of chlorastrolite. Most gem quality greenstones are very small, and it is rare to find one that is larger than a half inch. The largest gem quality stone is in the Smithsonian and measures 1.5 by 3 inches.

Since it is illegal to search for these gemstones on Isle Royal, the best localities now are the waste rock piles located in the old Keweenaw Peninsula mines in the western Upper Peninsula of Michigan. Some possible search sites include the Central, Central Exploration, Cliff, Phoenix, Mandan, and Delaware mines as well as some of the shoreline outcrops near Eagle Harbor.

MINERAL OF THE MONTH: June 2012 – Moqui Marbles


The mineral of the month for this update is an unusual type of iron concretion called Moqui Marbles. These round rocks form in the Navajo Sandstone formations spread across northern Arizona, northwest Colorado, and Utah. Most are found in the numerous national parks in the area, so they can no longer be collected. I recently purchased a large quantity of moqui marbles that were collected legally many decades ago.


The wide range of colors exhibited by the Navajo Sandstone reflect alteration by groundwater fluids over the last 190 million years. The different colors are caused by the presence of varying mixtures and amounts of iron minerals such as hematite, goethite, and limonite. These minerals fill the pore spaces between grains of sand, causing the variation in colors in the sedimentary layers. The iron in these strata originally arrived via the erosion of iron-bearing silicate minerals. Variations in the type and proportions of precipitated iron oxides resulted in the different crimson, vermillion, orange, salmon, peach, pink, gold, and yellow colors of the Navajo Sandstone.

The Navajo Sandstone is also well known for its iron concretions. They are believed to represent an extension of Hopi Native American traditions regarding ancestor worship (“moqui” translates to “the dead” in the Hopi language). Informally, they are called “Moqui marbles” after the local proposed Moqui native American tribe. Thousands of these concretions weather out of outcrops of the Navajo Sandstone within south-central and southeastern Utah within an area extending from Zion National Park eastward to Arches and Canyonland national parks. They are quite abundant within Grand Staircase-Escalante National Monument. The Utah concretions formed around 25 million years ago when minerals precipitated from groundwater flowing through much older Navajo sandstone.

The iron oxide concretions found in the Navajo Sandstone exhibit a wide variety of sizes and shapes including discs, buttons, spiked balls, hollow pipes, round spheres, and others. Although many of these concretions are fused together like soap bubbles, many more also occur as isolated concretions, which range in diameter from the size of peas to baseballs.


These concretions are regarded as terrestrial analogues of the hematite spherules, called Martian “blueberries” or more technically Martian spherules, which the Opportunity Rover found on Mars.

Many people like to buy Moqui Marbles in pairs. The “male” spheres have ridges whereas the “female” spheres are smoother. Used together they balance the masculine and feminine energies. They are believed to be among the most energetic stones on earth. Their major properties are claimed to be cleansing, relaxation, and they provide a great boost to meditation.

MINERAL OF THE MONTH: March 2012 – Mexican Crazy Lace Agate

The mineral of the month for this update is Mexican Crazy Lace agate. Crazy lace agate, also known as Mexican agate, is an attractive, multi-colored agate that is patterned like a beautiful, multicolored paisley cloth. It is found in Northern Mexico in the state of Chihuahua. This is the largest of the 31 Mexican states – slightly larger than Great Britain.

Although Mexican agates were first documented in 1895, it wasn’t until some 50 years later that a few American rockhounds found a few small agates not far from the newly constructed highway they were travelling between Ciudad Juárez and Ciudad Chihuahua. Many different types of agates are found in this region, mostly in isolated deposits within andesites, rhyolites, and ash flow tuffs that range in age from 38 to 44 million years old. The only exception is Mexican Crazy Lace Agate which is uniquely found in a limestone layer of Cretaceous age (90 to 65 million years old). The Crazy Lace deposit is located southwest of Villa Ahumada.

In ancient times, this agate was worn to placate the gods, and to give courage. It will improve eyesight, illuminate your mind, allow you to be more eloquent, and give vitality. It keeps the wearer well-balanced, focused, and improves ability to accomplish goals and overcome adversity. Like other agates and silica rocks, Crazy Lace agate is a good general healing stone.


MINERAL OF THE MONTH: January 2012 – Mary Ellen Jasper

The mineral of the month for this update is Mary Ellen Jasper. Unless you live in Minnesota, you may not be familiar with this interesting form of microcrystalline quartz. This rock formed more than two billion years ago in the area that is now the Mesabi Iron Range in Northern, Minnesota. At that time one of the early life forms evolved in the ancient seas. These blue-green single-celled cyanobacteria contained chlorophyll and were able to harvest the energy of the sun to photosynthesize and produce their own food. Energy from sunlight was used to split carbon dioxide into carbon and oxygen. The carbon was absorbed, becoming part of the growing organism, and the oxygen was released into the atmosphere. Prior to the evolution of cyanobacteria, there was almost no oxygen in the atmosphere. Once these organisms developed, they proliferated and helped to trigger drastic changes in the earth’s atmosphere, climate, and environment.

Some of the cyanobacteria lived in colonies that produced macro-scale structures called stromatolites. A drawing depicting what a stromatolite shoreline may have looked like during the latter part of the Archean period is shown below. Evidence of fossil stromatolite formations have been found throughout the world so these mushroom-shaped mounds dominated the shores of all the newly developing landmasses, including the area where the Mary Ellen Jasper developed.

The earliest stromatolite of confirmed origin dates to 2,724 million years ago. A recent discovery, however, provides strong evidence that microbial stromatolites extending as far back as 3,450 million years ago. These organisms were extremely resilient and adaptable, allowing them to be a major constituent of the fossil record for the first 3,500 million years of life on earth, with their abundance peaking about 1,250 million years ago.

Until the mid-1950s, scientists thought that stromatolites were long since extinct. That all changed in 1956 when living stromatolites were found in the Hamlin Pool located on the south end of Sharks Bay in Western Australia. Since then, live stromatolites have also been found in several sites in the Bahamas. Pictures of both are included below.

Stromatolites are stony structures built up by algae and cyanobacteria. The microbes live in gooey mats on the top surface of the structures. These mats trap fine sediments carried across them by tidal currents. As the mats fill in with sediments and become opaque, the microbes move upwards seeking sunlight. Stromatolites differ from normal fossils because they are formed by the activities of micro-organisms. They result from a combination of trapping, binding and precipitation of sediment.

One of the biggest impacts that stromatolites had on the earth was the release of free oxygen, which was a byproduct of their photosynthesis. When stromatolites first evolved, the earth’s atmosphere had less than one percent oxygen. After the stromatolites evolved, significant amounts of oxygen did not accumulate in the atmosphere right away because of the vast quantities of oxidizable materials in the earth’s crust as well as the dissolved eager-to-combine iron in the oceans. For more than a hundred million years, these materials absorbed any free oxygen that was produced. Mary Ellen Jasper developed not only from the remains of the stromatolites, but also from the oxidization of iron that was present in the area that is now northern Minnesota.

A few more pictures of Mary Ellen Jasper are included below. The first two pictures are of a thin polished slab. The first is displayed with front lighting and the second with back lighting.

Coincidently, I just polished a piece of Mary Ellen Jasper for a customer a couple of days ago. Here is a picture of that specimen.

MINERAL OF THE MONTH: Winter 2011 – Argentina Condor Agate

During the past several years Argentina has become one of the new hot beds for agate prospecting. The condor agate was introduced to the U.S. market in 1992 by the former Argentinean actor, Luis de los Santos. Since then he discovered the puma agate in 1993, the crater agate in 1997, and more recently the Black River agate. Prospecting and mining these agates is difficult. Not only are the locations extremely remote, but the work must be done at elevation where the weather is often not cooperative. Luis named this agate after the large Condor birds that were flying over him during the trip that he discovered the agate. It is mined at over 6,000 feet. The site can only be reached by horse back.