MINERAL OF THE MONTH: September 2011 – Petoskey Stones

In 1965 the state stone for Michigan was designated as the Petoskey stone. Petoskey stones are fossils of coral colonies called Hexagonaria. “Hex” comes from hexagon, because the coral is usually a six-sided polygon. The coral grew 350 million years ago, 150 million years before the dinosaurs! It grew only in shallow tropical salt-water seas where lower Michigan is now. The coral stacked in layers, then fossilized into rock.

The Petoskey stones found on the beaches of Lakes Michigan and Huron were formed as a result of glaciation, in which sheets of ice plucked stones from the bedrock, grinding off their rough edges and depositing them primarily in the northern portion of Michigan’s Lower Peninsula. In some areas of Michigan, complete fossilized coral colony heads can be found. The formations and specimens found inland tend to be rougher since they have not been weathered as much by the wind, water, and sand from the shoreline. The movement of the frozen lake ice acting on the shore during the winters is thought to turn over stones at the shore exposing new Petoskey stones at the water’s edge each spring.

Similar fossil corals occur in a variety of locations; however the name Petoskey stone should only be applied to those from Michigan which exhibit a six-walled coral structure that has the distinctive “eye” pattern within each cell.

When ancient glaciers pressed down on the center of the Lower Peninsula, the layer of fossilized coral rose up to form a ridge and created the dish shaped “Michigan basin”. The exposed layer of rock is where Petoskey stones come from. Petoskey stones are found in the Gravel Point Formation of the Traverse Group. They are fragments of a coral reef that was originally deposited during the Devonian period. When dry, the stone resembles ordinary limestone but when wet or polished using lapidary techniques, the distinctive mottled pattern of the six-sided coral fossils emerges.

It is sometimes made into decorative objects. Other forms of fossilized coral are also found in the same location including the closely related favorites. Favosites is an extinct genus of coral characterized by polygonal closely-packed corallites (giving it the common name “honeycomb coral”). The walls between corallites are pierced by pores which allowed transfer of nutrients between polyps.

The name comes from an Ottawa Indian Chief, Chief Petosegay. The city of Petoskey, Michigan, is also named after him, and is the center of the area where the stones are found. According to legend, Petosegay was the child of a descendant of French nobleman and fur trader, Antoine Carre and an Ottawa princess. Petosegay, meaning “rising sun”, “rays of dawn” or “sunbeams of promise”, was named after the rays of sun that fell upon his newborn face. In keeping with his promising name, Petosegay was a wealthy fur trader who gained much land and acclaim for himself and his tribe. He was remarked upon to have a striking and appealing appearance, and spoke English very well. He married another Ottawa, and together they had two daughters and eight sons. In the summer of 1873, a few years before the chief’s passing, a city began on his land along Little Traverse Bay. The settlers christened the newborn city Petoskey, an anglicized form of Petosegay.


MINERAL OF THE MONTH: Spring 2011 – Petrified Wood

Petrified Wood can be thought of as a type of fossil. Fossils are formed from a petrification process, which is a geology term describing the method by which organic living material is converted into stone. Usually, this happens when the organic remains are buried in lava or sediments before they can decay. Petrification can take place in two related ways: replacement and permineralization. Replacement occurs when water dissolves the original hard parts and replaces them with mineral matter. The most common replacement minerals are calcite, silica, pyrite, and hematite. When the original organism is replaced quickly, the fossil usually loses the detailed structure, leaving behind just the original shape. Permineralization occurs when ground water carrying dissolved minerals infiltrates the microscopic pores. The minerals in this case replace the detail of the original organism.

The most famous example of petrification is the Petrified Forest in northern Arizona. However, this is not the only area known for petrified wood since it is also found in nearly every state and in many foreign countries. It is not known for certain who the first Europeans were to see the great display of petrified wood in Arizona. It was probably Spanish explorers during their expeditions in the 1500s. The earliest written record, however, dates from 1851 when an army officer mentioned petrified wood in a report.

At one time the northeast part of Arizona was lowland with numerous rivers and streams. In the basin there was a lush forest with conifer trees up to nine feet in diameter and over 200 feet tall. During the Triassic Period (200 to 250 million years ago), the area that is now Arizona was located near the equator. At that time, all of the continents had combined to form one super-continent called Pangea. Over time, trees in the area died. Rivers deposited the deceased trees in their flood plains or buried them in streambeds. Most of the trees decomposed, but a few were buried so deep that there was not enough oxygen to allow decay. To the west of this area were massive volcanoes that spewed ash into the atmosphere. Wind currents carried the ash and deposited it with the silt that buried the trees. Ground water dissolved silica from the ash and carried it into the buried logs. Over time, the silica in solution either replaced cell walls, crystallizing as quartz, or deposited in the air spaces within the wood tissue. This petrification process explains how cell structure, annual rings, and other features of the original trees were preserved.

As the petrification process continued, other minerals combined with quartz to create the brilliant rainbow of colors often found in petrified wood. In some cases minerals infiltrated later during the millions of years of burial as a secondary deposit in the cracks, checks, or other openings in the petrified or partially petrified wood. Iron oxides produced the great variety of shades of red, brown, and yellow. The black color was probably due to manganese oxide or carbon.

At first, the now petrified trees remained buried under 3,000 feet of sediment. However, around 60 million years ago this area was uplifted along with the Rocky Mountains. With the uplifting came erosion from streams and rivers that removed the overburden layers, exposing the petrified trees.

Some scientists classify petrified wood as agate; others do not. The determination comes down to two issues. First, the type of quartz needs to be identified. In some cases, most of the quartz in petrified wood is macrocrystalline, although there may also be some microcrystalline replacement. For those specimens that contain mostly microcrystalline crystals, the second issue is whether petrification was a replacement process, or whether it may have involved an agatization process. If the wood tissue was directly replaced by silica so that the wood fibers defined the structure, then petrified wood is not agate. If, on the other hand, each of the individual pore spaces self-organized and filled independently, thus creating individual agatized pockets, you could then classify petrified wood as a colony of agates. Depending on the specimen, there could be some of both structures.


MINERAL OF THE MONTH: Fall 2010 – Ammonite Fossil

The mineral of the month is the ammonite (Ammonoid) fossil. Ammonites are an extinct group of marine invertebrate animals from the Cephalopod class. These fossilized mollusks are more closely related to living cephalopods such as octopuses and squid, than they are to the modern nautilus, which has a similarly shaped shell. Two pictures of ammonite fossils are below. I took the first photo and Tom Shearer took the other two photos (Figures 135 & 136 in the new agate book).

Ammonites are excellent index fossils. Depending on the species of ammonite found in a rock layer, geologists can specify the geologic time period for that layer.

The name ammonite was inspired by the spiral shape of the fossilized shells, which somewhat resemble tightly-coiled rams’ horns. The Egyptian god, Ammon, was typically depicted wearing rams’ horns.

Eight different orders of ammonites are known to have existed, ranging from 400 million to 65.5 million years ago. They ranged in size from a fraction of an inch to over 7 feet in diameter. After being born, they fed on plankton and quickly assumed a strong protective outer shell. They also grew quickly with the females growing up to 400 percent larger than the males because they needed the extra space for egg production.

Because ammonites are extinct, little is known about their way of life. Their soft body parts were very rarely preserved in any detail. It is thought that depending on the species, they lived in a variety of ocean environments. Some probably lived in open water, while others survived at the bottom of the ocean. It is also believed that ammonites may have avoided becoming a predator’s dinner by squirting ink, much like modern cephalopods.

The soft body of the organism occupied the largest segment of the shell at the end of the coil. The smaller sections were walled off which allowed the animal to maintain its buoyancy by either filling the chambers with gas, or emptying the sea water out of these chambers. Thus the smaller sections of the coil would have floated above the larger sections. As it grew, it added newer and larger chambers to the open end of the coil.

In medieval Europe, fossil ammonites were thought to be petrified coiled snakes. These fossils were often called “snakestones” or “serpentstones.”

MINERAL OF THE MONTH: August and September 2010 – Polyhedroid Agate

The mineral of the month for August/September is the Polyhedroid Agate. The unique polyhedroids are strange looking agates with geometric shapes that have smooth flat sides. Their shape appears more angular than the typical almond-shaped agate, but there is no consistency to the angles of formation. Most are either triangular or trapezoid and hollow, with the bands running parallel to their shape. Although no one knows for sure how these agates formed, there are at least two theories. Some believe that polyhedroid agates are pseudomorph replacements of other, more angular crystals. Another hypothesis is that they formed in spaces between other crystals, perhaps calcite. Unlike the rest of the Brazilian agates found in the southern part of the country, these unusual agates are found at a single site in the northern state of Paraiba. There has been no mining of polyhedroids in recent years. Most were mined during the 1970s. Believe it or not, a few polyhedroids have also been found in the Lake Superior region and in Wyoming.


MINERAL OF THE MONTH: July 2010 – Mookaite Jasper

Mookaite jasper is a sedimentary rock formed when ancient sea bed sediment was jasperized into a mottled and beautiful quartz rock. This decorative rock, which is often used in jewelry, has truly beautiful patterns. It is only found in Australia and has the same earthy colors as those used by Aborigines in their art work. It is fossiliferous made up of the skeletal remains of radiolaria that had skeletal structures made of opaline silica. Billions of their remains were deposited as sediment in the shallow areas of ancient sea beds. When the sea levels declined, the sediments were cemented into solid rock by additional silica carried in groundwater. Colors were added by the remains of other marine organisms as well as minerals in the water. Mookaite includes various forms of silica including jasper, chert and chalcedony. Although most Mookaite contains just the chemical remains of the radiolaria, occasionally impressions of ammonites and other organisms have also been preserved.

Mookaite jasper (also known as mookite, mookalite, mookerite, moakite, moukalite and moukaite) is named after Mooka Creek which is located in the Kennedy Range near Gascoyne Junction, about 100 miles inland from the coastal town of Carnarvon in Western Australia.

This is a popular jasper because of its extreme range in color. It can include many bright colors including red, purple, tan, snow white, ivory white, pink, gold, black, and yellow.

Metaphysical Properties

Mookaite is an extremely soothing stone and can help to relieve stress, tension, and the negative effects of emotional trauma. It can help you to regroup when life gets complex and can also help to enhance strength and endurance to positively affect wellbeing. Some believe that it helps to stimulate youthful exuberance, which in turn can help to slow the aging process. It can also promote courage to help you to face new challenges. Mookaite’s healing powers can benefit glandular, digestive, circulatory, and immune systems.

MINERAL OF THE MONTH: June 2010 – Nantan Meteorite


A meteorite is a piece of a meteor that reaches the earth’s surface. Most of the meteorites found today were not witnessed when they fell. The Nantan meteorite fall is an exception. In May of 1516 during the reign of the Chinese Emperor Zhengde, the meteor hit the atmosphere above the Nantan province of China and broke into thousands of pieces that ranged in size from .35 ounces to 4,400 pounds. It is thought that the total weight of the meteor before it broke apart was around 20,900 pounds! The meteorite pieces were spread over an area that is approximately16 miles long and five miles wide.

A few years ago it was much easier to purchase Nantan meteorite specimens. Today they are very expensive costing around a dollar a gram. The one pictured below was acquired at the Denver Gem and Mineral Show around ten years ago. It weighs a little over one pound. Most of the Nantans that you see for sale are heavily oxidized. This is because large portions of the meteor was composed of iron oxide (rust) and not iron. By contrast this specimen came from the core of the meteor and is composed of approximately 98 percent metal. In addition to 92 percent iron and 6 percent nickel, more than ten minerals have been found in the Nantan meteorites including kamacite, taenite, plessite, scheribersite, triolite, graphite, spherlite, sideroferrite, dyslytite, cliftonite, and lawrencite. The Widmanstattern cross-banded woven pattern that is typical in iron meteorites is obvious in polished Nantan pieces.

What is interesting about these meteorites is that no one paid attention to them until 1958 when China needed a lot of steel to push the country a “Great Leap Forward”. During this period, everyone was told to look for iron ore. Farmers knew about the iron rocks and had found that they were so pure that they could not be melted in a regular fire. They reported the specimens to the government and scientists were able to prove that the “iron ore rocks” were in fact iron meteorites.

P.S.—I didn’t realize until today that I featured Kentucky agate twice in the last few months as the selected mineral. Chalk it up to having too many things to do and not enough time to do them. I apologize for the duplication.

MINERAL OF THE MONTH: April/May 2010 – Kentucky Agate

The mineral of the month is Kentucky agate. It became the state stone for Kentucky in 2000. This agate is quite unique in its coloration. It most typically has bands of yellow, black, and gray. Occasionally there are sections of blue, red, pink, or orange. This distinctive agate was formed in shale and siltstone in the Borden layer of the Mississippian geologic period between 225 and 375 million years ago. They can be found in Estill, Jackson, Lee, and Powell counties. Hunters find them most often in the Buck, White Oak, and Middle Fork Creeks. Due to the hilly terrain, rains cause the creeks to rise sometimes 10 or 15 feet, depending on the location. The rushing water washes the agate nodules out of the sedimentary layers. Since most Kentucky agates have thick husks, successful hunters use a rock hammer or other metal object to hit the rocks in the river. The silica rocks have a “ping” sound, as compared to other river rocks that have more of a “thud” sound. Not all silica rocks found in the creeks are agate, however. There are also silicified fossils, geodes, and quartz balls.

The photos included in this update were taken by Tom Shearer. These are three of the photos from the new agate book Please see the gift shop section of this web page for more information about the new agate book.


MINERAL OF THE MONTH: March 2010 – Fossilized Dinosaur Bone

This month’s featured mineral is fossilized dinosaur bone. For the new agate book, Tom Shearer borrowed samples of fossilized bone from Mark Anderson (www.differentseasonsjewelry.com). The specimens pictured below come from Utah. Although they certainly have agate pockets, technically the fossilization process is different than the agatization process. Therefore, these specimens are classified as fossilized dinosaur bones that have agate pockets.

You might wonder how these bones become fossilized in the first place. The process happened as follows:

  1. After their death, dinosaurs became buried in mud or sand.
  2. The softer tissues decayed and rotted. However, the harder parts like bones and teeth did not rot and became encased in the sediment.
  3. In time mineral-rich water seeped into the bone. The process of fossilization occurred in two different ways. Either the original organic material was replaced with other minerals (permineralization), or tiny spaces in the bone filled in with minerals (recrystallization). In the case of the fossil photographs included below, agate filled in some of the spaces.
  4. This process causes the bone to turn into a fossil. The fossil has the same shape as the original bone, but chemically has been converted to rock!


MINERAL OF THE MONTH: Fall 2009 – Fairburn Agate

The Fairburn Agate is the state stone in South Dakota. It is named after the town from which the first agates were found: Fairburn, SD. This is an unusual agate, in that they formed in sedimentary rock, rather than igneous rock in which most other agates formed. This sedimentary genesis probably contributed the minerals which cause the brilliant color of Fairburns. These agates are also known for the extreme color contrast in the banding. The agate bands typically have sharp peaks, often resembling a maple leaf pattern. Most Fairburns have concentric fortification banding.

These agates are found in the grassland and badland areas surrounding the Black Hills, located in the southern part of the state. It is believed that the sedimentary rock matrix in which the agates formed were uplifted when the Black Hills formed. As the limestone eroded, the hard agates eroded out. As they moved downstream with eroded material, many of the Fairburns were rounded and river polished.

The value of a Fairburn agate is determined by size (most are small—only a few ounces), color, quality of banding, number of bands, shape of the fortification pattern, and condition (fracture free). As far as I know, a Fairburn agate still has the record for the highest priced agate sold on EBay. Several years ago, a Fairburn agate sold for $13,000.

MINERAL OF THE MONTH: September 2009 – Banded Iron Formation

In the iron-rich areas around Lake Superior, there are deposits of banded iron formation. In addition to being called banded iron formation, many rockhounds label specimens as Jasperite, Jaspite, or Jaspelite. This is a distinctive type of rock found in Precambrian sedimentary rocks. The structures consist of repeated layers of iron oxides, either magnetite or hematite, alternating with bands of iron-poor chert or jasper. Some of the banded iron formations date back to 3 billion years ago. However, most formed around 1.8 billion years ago: others are much younger.

The total amount of oxygen locked up in the chemical bonds of the banded iron beds is believed to be perhaps twenty times the volume of oxygen present in the modern atmosphere. Banded iron beds are an important commercial source of iron ore. They exist in several areas of the world including Western Australia, Minnesota, and the Upper Peninsula of Michigan.

Scientists believe that the banded iron layers formed in sea water as the result of oxygen released by photosynthetic bacteria. The oxygen combined with dissolved iron in the oceans to form insoluble iron oxides, which precipitated out forming a thin layer at the bottom of the ocean. It seems that the amount of oxygen varied, perhaps by season, so that at other times chert or jasper formed intermediate layers when oxygen levels were lower.

The picture of the Jasperite boulder included above is in a boulder garden at my friends house (Kat and Ken), who live near Aitkin, MN. The other photo I took of a specimen for sale at the Ishpeming Gem and Mineral Show earlier this month.