Exploring the Different Types of Coal

Exploring the Different Types of Coal

Coal is mined for use in the production of the energy that we use to power our homes and offices. These rocks may look like simple black, lumps, but they are unique in their content and ability to produce energy. Interestingly, there are four types of coal making it a diverse and fascinating rock.

How Coal Forms

Coal is a brownish-black colored rock that creates energy as it burns. Coal forms from dead plant matter that builds up in swampy, watery areas of our plant. As the shrubs and trees die in the swamp, they fall and stack up on top of each other. Over time, these dead plants, form layers and start to decompose, or rot, and break down and form a material called peat The peat is buried deeper and deeper below the land surface as more layers of plant matter, rocks, and sediment deposit on top. As the layers become deeper underground, the pressure and heat rise, and change the peat into coal. Coalification is the name of the coal creation process.

Types of Coal and Their Ranking

Lignite Coal - Sedimentary Rock

There are 4 main types of coal. Coal varieties are both sedimentary rock and metamorphic rock. The three types of sedimentary coal are lignite, sub-bituminous, and bituminous. Anthracite is a variety of metamorphic coal. Experts rank coal by the amount of carbon inside the coal and the amount of heat that it can produce when it is burned.

The ranking order for coal is Lignite coal being the lowest rank, then Sub-Bituminous, Bituminous, and then Anthracite, which is the highest rank of coal and the most desirable variety for both energy companies and Santa!

Lignite Coal

Lignite coal has 25% to 35% carbon and produces the lowest energy of all coal. It is the youngest coal and spends the least amount of time underground being exposed to high heat and pressure. It is one of the easier varieties to identify because it is very lightweight and looks very dull. Lignite Coal is also very crumbly and has a lot of moisture compared to other types.

Bituminous Coal - Sedimentary Rock

Sub-Bituminous Coal

Sub-Bituminous Coal has 35% to 45% carbon. It produces more energy than Lignite but less than Bituminous coal.

Bituminous Coal

Bituminous Coal is the most abundant type of coal in the United States. It has 45 to “86% carbon. It is much harder than lignite coal, so it is not crumbly. But it still has a dull appearance.

Anthracite Coal

Anthracite Coal - Metamorphic Rock

Anthracite Coal contains 86 to 97% carbon and typically produces the most heat of all types of coal. One of the best ways to tell the difference in this type of coal is its luster. Anthracite tends to be shinier than all other types of coal. Interestingly, in the United States, northeastern Pennsylvania is the only location with anthracite. Other coal varieties are found in several states.

Coal is important to our way of life, but it is not a renewable resource. It takes millions of years for the coalification process to happen and create these different types of coal.

Ask-a-Geologist #22: What are some common household uses of rocks and minerals?

In this video answer, we discuss several types of rocks and minerals and their common household uses. There are probably some you know and maybe a few you do not. We talk about one you probably use every day and one you may even eat. Watch the video and see if you can come up with other rock and minerals in your area.

What are Common Household Uses of Rocks and Minerals?

Teddy wrote to us and asked, what are a lot of the common household items and items around my town that I might see that are made from rocks and minerals?

Pencil tips are made of the mineral graphite.

Hi, Teddy, believe it or not, rocks and minerals are used in lots and lots of things that you can find around your house. Now, one of the most common is a pencil. Everybody calls the tip of this the pencil lead, but this is actually graphite.

Graphite is a mineral made out of carbon, and that is actually what you use to write with when you’re using your pencil. Now, there are lots of other uses for minerals and rocks.

Uses for Azurite, Gypsum, and Halite Minerals

Some of the minerals that are most interesting are azurite and gypsum. Azurite has a beautiful blue color and is often used to make dyes, specifically blue-colored dye that is used for different products like paints and clothing. The next useful mineral is gypsum. Look at the walls of your house. Often, the plaster in the drywall around your house has some gypsum inside.

Azurite mineral is used to make blue dyes.

Another mineral that you very likely have in your kitchen and possibly use every day, is halite. Halite is another name for table salt. Salt is made of sodium chloride and it is most commonly used to season the food you eat. Salt can be an important part of cooking, cleaning, and even beauty products. Many people do not realize that table salt is a very important mineral.

Uses for Granite and Scoria Around Your Home

Many rocks are used as building materials. One of the most common rocks that you will find in your home or in buildings around town is granite. Granite comes in different colors from white and black to pink and black. It has a beautiful speckled appearance and it is very strong and durable making it a very popular choice for construction. Granite is often found inside houses as countertops, wall covering, and as tiles. Outside, it may be as crushed materials for roadbeds or as decorative stone in landscaping beds.

Scoria Igneous Rock is uses in landscape beds.

Scoria is another very popular rock that you may see outside of your house or a building in town. Many people call scoria “lava rock,” because it formed from a volcano and is usually red to dark brown in color. Scoria is very popular as a landscape material in the western portion of the United States. Consequently, you may see this type of rock scattered around the landscape beds around the plants because it is durable and helps to protect plants from erosion due to wind and rain. Scoria has a unique texture with many holes in the rock which also gives the rock an interesting appearance in your home.

Other Common Household Uses for Rocks and Minerals

Below is a list of different uses for other common rocks and minerals often found around your own home. See if you can find any of these in your home or school and then leave us a comment below about what you find.

Household Uses of Minerals

  • Talc -Baby powder.
  • Graphite – Pencil tips.
  • Lepidolite – Lithium content – Medicines may use lithium.
  • Beryl – Jewelry.
  • Epidote – Jewelry.
  • Rhodonite – Jewelry.
  • Malachite – Jewelry.
  • Amethyst – Jewelry and decor items.
  • Amazonite – Jewelry.
  • Garnet – Jewelry.
  • Limonite – Yellow & brown dyes and pigments.
  • Azurite – Blue dyes and pigments.
  • Quartz – Prisms, lenses, gauges, glass, paints, and abrasives.
  • Calcite – Microscopes, metallurgy, fertilizers & chemical industry.
  • Fluorite – Enamels, cooking utensils, telescopes, camera lenses.
  • Gypsum – Paints, tile, drywall, blackboard chalk, fertilizer, Plaster of Paris.
  • Halite – Salt for food preparation and in the chemical industry.

Household Uses of Igneous Rocks

  • Granite – Roadbed construction material, countertop, wall tile.
  • Scoria – Flower beds as landscaping stone.
  • Pumice – Foot smoothing stones, soap.

Household Uses of Sedimentary Rocks

  • Sandstone – Building materials and decorative accessories such as coasters and garden items.
  • Limestone – Building materials and decorative accessories statues, and garden decor.
  • Coquina – Building materials and decorative accessories such as garden decor.
  • Bituminous Coal – Fuel source.
  • Lignite Coal – Fuel source.

Household Uses of Metamorphic Rocks

  • Anthracite Coal – Fuel source, metamorphic coal gives off the most heat of any coal during the burning process.
  • Gneiss – Common building materials and decorative boulders.
  • Marble – Common building materials and decorative boulders. Common for statues carvings.
  • Slate – Flooring and roofing material, blackboards.

Leave a comment here or on our social media channel about your favorite items that are made from rocks and minerals. If you have an ask-a-geologist question for us, you can email us at rockinfo@minimegeology.com. We would love to hear from you.

Ask-a-Geologist #21: What types of tools does a geologist use in their daily job?

Welcome to Mini Me Geology’s Ask-a-Geologist question-and-answer video series. Today, in AAG #21, we answer Carrie’s question about the types of tools that geologists use to do their daily jobs.

Geologists use a variety of tools from hand lenses, acid, steel files, your own fingernail, specialty rock hammers, safety goggles, Brunton compasses, streak plates, and waterproof notebooks. Watch and learn a little about the geologist’s tools from Mini Me Geology owner and geologist, Tracy Jones.

We hope you like our video series. If you have questions about rocks, minerals, or other geology topics, send us an email at rockinfo@minimegeology.com.

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A Rock Weathering Experiment You Can Do In Your Kitchen!

Rock Weathering Experiment in Your Kitchen

Every day, rocks are subjected to wind, rain, and other mechanical processes that cause them to break down into smaller pieces and different forms. This process of weathering is part of the rock cycle and causes sedimentary, igneous, and metamorphic rocks to break down into smaller sediments and soil-sized particles. You can learn about rock weathering right in your own kitchen! Try this fun experiment to learn more about the mechanical weathering of rocks and post your results in the comments below.

Supplies for the Rock Weathering Experiment

  • Plastic Wrap
  • Clay
  • Water
  • Hand Magnifier
  • Notebook
  • Pen
  • Camera (optional)

Steps for the Rock Weathering Experiment

Step 1: Moisten the clay with a small amount of water. You want the clay to absorb as much water as possible without being dripping wet. Add a small amount of water to the clay and knead it until the water is absorbed, repeating until the clay is saturated.

Step 2: Divide the clay into two equal pieces and roll it into a ball or form a square.

Step 3: Wrap each piece of clay in plastic wrap.

Step 4: Place one piece of clay into the freezer and leave the other piece on a table or counter. Let the clay stay in the freezer overnight.

Step 5: The next day, remove the clay from the freezer and unwrap both pieces. Place the two balls of clay side-by-side and observe your results. Do the clay pieces look different after one day and then over time? If so, how? Write about your findings in a notebook and take pictures of the results after each day to see how the clay rock changes.

Step 6: Wrap each clay piece back up and put the one piece back into the freezer and repeat for several days. Observe the clay pieces each day and see how the cracks change over time.

Experiment Observations

The clay from the freezer should have some cracks. Examine the clay with a hand magnifier to get a closer look at the cracks. The cracks result from the freezing and expanding water just like a rock that has water freezing in holes or existing cracks in the rock. Over time, the freezing and expansion of rainwater will cause a small crack in a rock to become big and allow the rock to split. If the frozen clay does not crack after several days, repeat the experiment with more water in the clay.

You can find more great activities like these in our Rock Cycle Kit and Rock Detectives Kits!

The Geology of the Blue John Cavern, home of the Famous Blue John Fluorite

The Blue John Cavern is located in the Peak District in England near the town of Castleton, Derbyshire. This unique are is the home of the world-famous Blue John Fluorite. There are two caves in the area that have this unique fluorite – the Blue John Cavern and Treak Cliff Cavern.









Area Geology Near the Blue John Cavern

The area is formed of limestone strata that deposited in deep ocean waters millions of years ago. Layers of shale and some gritstone, a type of coarse sandstone common in this area, covered the limestone. A generalized diagram of the strata is shown here.

Blue John Cave Formation

Over time, the layers buckled from folding and faulting. The cavern itself formed when glacial meltwaters flowed through the area forming the valleys and seeping into cracks in the limestone strata and dissolving portions of the limestone. As the meltwaters dissolved the limestone, underground rivers formed which washed out corridors and large underground rooms which became the network of the Blue John Cavern.

Blue John Fluorite

Fluorite is a unique mineral that can be found in more colors than any other including blue, red, purple, yellow, green, or white. Fluorite has low to moderate hardness, a white streak, and a glassy luster. Fluorite often forms in hydrothermal (hot water) veins inside other rocks and is often found with other minerals like galena, calcite, quartz, and sphalerite. Interestingly, fluorite crystals will fluoresce under ultraviolet light.

The Blue John Fluorite is a banded white and purple variety of the mineral shown here.

Adventures at the Blue John Cavern

The Blue John Cavern area is the setting for much of the book, Blue John’s Cavern, a time-travel, adventure novel for kids age 8 to 14. Join Emma and Brody as they travel through the magical Crystal Cave in search of the rare Blue John Fluorite! Read the book blurb on our website.

Blue John’s Cavern, and the entire Crystal Cave Adventures book series is available on our website. The series is also available wherever books are sold. Use this link to find your favorite retailer!


Blue John’s Cavern, and each of the four Crystal Cave adventures novels has a companion Activity Book full of puzzles, story quizzes, online games, fun geology facts, behind-the-scenes book details and creative writing activities! You can find the activity book on our website and everywhere books are sold.

The Fascinating Bay of Fundy

The Bay of Fundy is a geologically fascinating location in North America. I visited the bay in summer 1990 with my college geology department from Furman University.

Where is the Bay of Fundy?

The Bay of Fundy is approximately 174 miles long and located along the eastern coast of North America in the provinces of New Brunswick and Nova Scotia in Canada and on the northern side of the Gulf of Maine. I visited the Bay of Fundy during a college field trip and found the formation of the area fascinating. The bay is along the Atlantic Ocean and is subject to intensly dramatic tidal changes.

Tidal Changes at the Bay of Fundy

The fascinating tidal changes at the Bay of Fundy are worth an in-person visit one day. The photo to the right shows an area of the Bay of Fundy at low tide. You can see people walking along the low area.

The 12.5-hour tide cycles show a dramatic increase and decrease in the water level of the Bay. During this phenomenal change, over one billion tons of water flow into and out of the Bay of Fundy.

The highest recorded change between low and high tide was 54.5 feet, documented at Burncoat Head in Nova Scotia. Typical tides changes in the Bay are generally up to 49 feet per cycle and are most pronounced near the Minas Basin in Nova Scotia, Canada. The photo above shows the same area of the Bay of Fundy approximately 12 hours later at high tide. The area where the people were walking at low tide is completely flooded.

Geology and Biology of the Bay of Fundy Area

The Bay of Fundy is home to many birds, fish, and marine animals such as whales, dolphins, porpoises, fish, seals, and seabirds. The geology of the area consists of sandstone, 200 million-year-old basalts that formed as statues and cliffs, zeolites, and semi-precious stones including amethyst, agate, calcite, copper, jasper, and coal. The rolling in and out of the tide reveals fossils from the ancient rock layers as erosion occurs from 350 million-year-old carboniferous rocks.   In one area of the Bay of Fundy, the Joggings Fossil Cliffs in Nova Scotia, you can see an almost complete fossil record of the Pennsylvanian Coal Age, approximately 299 to 318 million years ago. The site also shows fossils from ancient reptiles and some of the oldest dinosaurs in Canada.

Have you visited the Bay of Fundy? If so, share your thoughts on this amazing location.

Meet the Cave of the Crystals – Giant Selenite Crystal Cave in Mexico

The Cave of the Crystal or the Giant Crystal Cave is a fascinating selenite cave in Naica, Chihuahua, Mexico. The cave was discovered in 2000 by miners who were excavating a new tunnel in the Naica Mine at a depth of approximately 300 meters (980 feet) below the land surface. Earlier, in 1910, a smaller room named the Cave of Swords was discovered at a depth of 120 meters (390 feet), above the Cave of the Crystals. The Cave of Swords contains amazing one-meter long selenite crystals covering the walls.

How the Giant Crystal Cave was Discovered

Brothers Juan and Pedro Sanchez, miners drilling in the Naica Mine to excavate a new tunnel for the Industrias Peñoles mining company, discovered the Giant Crystal Cave in April 2000. They were drilling through the Naica fault at the time. The Naica Mine is well-known for rich deposits of silver, zinc, and lead.

Inside the Cave of the Crystals with Massive Selenite Gypsum

The giant cave is located in limestone rock and is shaped like a horseshoe. The floor of the cave is covered in selenite gypsum blocks. The large blades rise from the blocks and the cave floor at varying angles throughout the cave. The Cave of the Crystals has spectacular, giant selenite gypsum crystals that are over 12 meters (39 feet) in length and 4 meters (13 feet) in width and weigh over 55 tons! These gypsum crystals are among the largest in the world.

The cave itself is very difficult to explore. The natural air temperatures inside the cave reach 136F with 90 to 99 percent humidity making it impossible to visit until you have protective clothing and equipment. Studying the cave has been a difficult challenge for scientists given the high temperature and humidity. Researchers must wear special protective suits that keep the body cool and provide fresh air. Even with the cooling suits, scientists were only able to spend between 30 and 45 minutes in the cave during each visit.

Selenite Mineral Properties

Selenite is a form of the mineral gypsum, also called Satin Spar. Satin spar is a fibrous variety of the mineral which forms in long strands, which grouped, form “sticks.” The following are the properties of the selenite mineral.

  • Chemical Formula:  CaSO4 . 2H2O
  • Color:  Colorless or White
  • Crystal Shape:  Tabular. Also may form twins or rosettes.
  • Cleavage:  Perfect
  • Fracture:  Conchoidal to splintery
  • Hardness:  2 on Mohs Hardness Scale
  • Streak:  White
  • Luster:  Pearly to Glassy
  • Density: 2.308 g/cm3
  • Uses:  Paints, Tile, Drywall, Blackboard Chalk, Fertilizer, Plaster of Paris

How the Selenite Crystals Formed Inside the Giant Crystal Cave

Scientists believe that the selenite crystals in the Giant Crystal Cave took approximately 500,000 years to grow to their current size. The heat in the cave results from a pool of liquid magma located approximately two to three miles below the main room of the cave. The cave periodically floods with water rich in gypsum minerals. The heat from the magma allows the selenite crystals to form from the mineral-rich water. The crystals form very slowly over a long time, which is why the selenite sticks are so huge.

Add Selenite to Your Collection

Mini Me Geology carries beautiful samples of selenite sticks, natural gypsum, and gypsum roses. These are fantastic minerals to add to your collections and they make a great gift for rockhounds as well! If you have any questions about any of our gypsum samples, please let us know.

Happy 50th Birthday Earth Day!

Earth Day was born on April 22, 1970. Every year, on April 22nd, we celebrate the wonders of our Earth and all that it has to offer and everything that we can do to help preserve its beauty and resources.

The Rise of the Environmental Movement

Throughout much of history, the environment was not of great concern. In 1962, the book Silent Spring by Rachel Carson, a life-long scientist, shed light on the negative impact of toxic chemicals on our environment. Ms. Carson’s legacy is fascinating and I encourage you to visit her website to learn more about this pioneering woman in science.

Silent Spring was an overwhelming bestseller and gave rise to the environmental movement. The book promoted concern for the health of our world’s animals, plants, and ecosystems and illustrated the potential destruction from reckless pollution.

The First Earth Day

Senator Gaylord Nelson from Wisconsin founded the first Earth Day. He was inspired to promote the environmental movement after he saw the effects of a large oil spill in Santa Barbara in 1969. Building on the environmental awareness that was growing in the world, he used Earth Day as a platform to change the way that air and water pollution were addressed in the United States. Over 20 million people participated in Earth Day in their communities. The day, aimed at teaching others about how to care for the environment and raising general awareness, drew crowds of people from young to old all across the country.

Earth Day Over the Years

Since 1970, Earth Day has become a global event that is celebrated in over 193 countries. The Earth Day Network helps to plan activities throughout the world to continue to bring awareness to the importance of caring for our environment. You can learn more about the Earth Day Network and how to become involved on their website.

Careers in Environmental Science

As a geologist, I spent many years working in the environmental industry as a consultant to corporations and individuals. My work involved assessing properties that had possible pollution then designing and executing a method for clean-up. Whether the site has soil or groundwater contamination, I and the companies I worked with, ensured that our clients were able to achieve their goals of protecting the environment.

If you are interested in working in the environmental arena, geologists and biologists are always in demand. You can work as a consultant, for the government helping to ensure that pollution is properly handled, as a researcher studying the environment and ways to keep it healthy, and much more. The Cool Jobs in Geology article may help you on your path as a future scientist. As always, if you have a question about careers in geology or environmental, contact me and I will be happy to help you find the right path forward.

Happy Earth Day!

What is My Birthstone Mineral and What are its Properties?

What's My Birthstone?

For almost 2,000 years, humans associated crystals and gemstones with the months of the year. Today, we call them birthstone minerals or mineraloids each with its own distinct characteristics. What is your birthstone?

Garnet birthstones january

Garnet ”“ January’s Birthstone

January’s birthstone is Garnet. Garnets form in metamorphic rocks such as schist and are a family of minerals that are all similar. The gemstone garnets are typically a dark red color; however, the brilliant green variety of uvarovite is rare and very prized.

Amethyst Birthstone February

Amethyst ”“ February’s Birthstone

Amethyst is a common, purple form of the mineral quartz.  Fairly hard, amethyst is a 7 on the Mohs hardness scale which has a range from 1 (softest) to 10 (hardest).  Crystals of amethyst often form in clusters, which are also called druze, and also commonly form in geodes.  The purple color of amethyst is due to the presence of ferric iron (Fe3+) in the crystal and can range from light to dark color.  When heated, amethyst will turn brown, into citrine.

Amethyst is considered a semi-precious gemstone that is most commonly used in jewelry and for collecting.  Beautiful samples of amethyst can be found in Brazil, Mexico, Uruguay, Russia, India, Sri Lanka, and the United States.

Aquamarine ”“ March’s Birthstone

Aquamarine birthstone march

Aquamarine is a pale blue form of the mineral beryl.  These gemstone crystals can occur in such rocks as granite and pegmatite.

These beautiful gemstones have a glassy luster (shine) and are either translucent or transparent.  These properties make aquamarine a prized stone for all types of jewelry.

Diamond ”“ April’s Birthstone

Diamond is the birthstone for April

Did you know that pencil lead and diamonds are made of the same thing?  It is hard to believe but they are both made of carbon!  The carbon forms in different crystal types or shapes, which is why they are different.  Diamond is the hardest mineral, being a 10 on the Mohs hardness scale.

Kimberlite Rock Cores

Diamonds form in igneous rocks called Kimberlites and Lamproites. These igneous rocks are typically rich in the mineral olivine and derive from mantle rocks known as peridotites. The peridotite rocks melt deep below the surface (between 90 and 280 miles) of the Earth and then rise through cracks in the surrounding rock forming pipe-shaped intrusions. As the magma pushes its way through the cracks, some of the surrounding rocks break off and mix into the liquid magma.

Over time, (millions of years) the magma slowly cools into rock with large crystals. Sometimes these crystals include those wonderful gemstones we know as diamonds.  This photo shows cores of kimberlite rock from a drilling and exploration project. The rocks cores are stored these long, then boxes for examination. Geologists store the cores in order as they pull them from the ground so that they can view the rocks in one long string and see exactly what is present below the land surface.

The largest diamond ever found is over 7,000 carats which is about the size of your two fists put together.  Because of their beauty and strength, diamonds are used for a wide variety of products from jewelry to industrial cutting blades. The photo at the top left shows a group of different-sized diamonds which have been cut and faceted.

Emerald ”“ May’s Birthstone

emerald birthstone may

Emerald is a green form of the mineral beryl.  A hard gemstone, emerald, has a glassy luster (shiny) and is either translucent or transparent.  The elements chromium and vanadium give emerald its green color.  Emeralds are one of the most rare and prized gemstones and can be worth more than diamonds if they are pure.

Pearl ”“ June’s Birthstone

Pearl birthstone june

Pearls are beautiful organic gemstones that are formed in a variety of colors and shapes.  Pearls form inside of mollusk shells such as oysters and mussels.  This unique gemstone is made of primarily the mineral aragonite.  Aragonite is the mineral that lines the inside of the mollusk shell.  An organic substance, called conchiolin, is also known to line the inside of the shell.  When shell linings are made of aragonite and conchiolin together, it is called mother-of-pearl.

To form the gemstone pearl, a grain of sand must get trapped inside the mollusk shell.  The aragonite forms in circles around the sand grain.  It can take between 2 and 8 years for a large pearl to be formed.  Fairly soft, pearls are a 3 on the Mohs hardness scale and have a white streak. The luster, “pearly,” is often used to describe the look of other minerals with similar outward appearance. Pearls can form in both freshwater and saltwater and can be round and smooth to oblong and uneven.  Common colors of pearls include white, cream, and black; however, other colors such as blue, yellow, gray, green, light purple, and mauve can also be found.

Ruby ”“ July’s Birthstone

Ruby Birthstone July

Rubies are one of the hardest-known minerals.  With a hardness of 9 on the Mohs scale, it is only softer than a diamond.  Rubies are known for their beautiful red color.  A ruby is a variety of the mineral corundum.  The gemstone sapphire is also a variety of corundum and comes in many colors, except for red.  Rubies have a glassy, shiny, luster which makes them popular for jewelry.  These gemstones are commonly mined in Myanmar, Thailand, Kenya, the United States, Sri Lanka, and Tanzania.  In 2005, a 440-carat ruby was discovered and thought to be one of the largest gems found to date.

Peridot ”“ August’s Birthstone

Peridot Birthstone August

Peridot (“pair-a-doe”) is the August birthstone.  This lovely green gemstone is a variety of the mineral olivine. Peridot gets its green color from the presence of iron in the crystal’s structure.  The amount of iron present determines the intensity of the green color such that the higher the iron content the darker the green color. Interestingly, peridot is only found in the color green whereas most other minerals can be found in more than one color.

Although olivine is common in igneous and metamorphic rocks, the gemstone quality version is much rarer. Arizona has one of the more abundant sources of peridot but the quality is not as high as in other regions, such as Egypt, Myanmar, Burma, and Pakistan, which have smaller amounts of the gemstone. The Smithsonian Institution has a gemstone from Egypt that is over 310 carats, which is the largest peridot ever found.

Sapphire ”“ September’s Birthstone

Sapphire Birthstone September

Sapphires are a blue form of the mineral corundum.  When a corundum is red, it is called a ruby.  Corundum is one of the hardest known minerals with a hardness of 9 on the Mohs scale.  The gemstone sapphire is usually known for its spectacular blue color, but can also come in many other colors, except for red.  Sapphires have a glassy luster which makes them shiny and are most commonly used for jewelry.  One of the largest sapphires in the world is called the “Logan Sapphire” and is over 422 carats.  Many beautiful specimens of sapphire are mined in Myanmar, Madagascar, Kashmire, Sri Lanka, and Australia.

In the United States, sapphires and rubies can be found in the area of Franklin, North Carolina, which is a popular area for mines.

Opal ”“ October’s Birthstone

Opal Birthstone October

Opal is a unique mineral because it forms as a gel in the cracks of many different types of rocks. However, opal is most often found in rocks like basalt, rhyolite, sandstone, and limonite.  High water content is a trademark of the opal.  Up to 20% of opal can be water.  Opals come in a wide variety of colors including white, reds, greens, pinks, browns, and blues to name a few.  Most opals do not have a specific shape, however, “precious” opals which are used for jewelry do have a round structure to them. Almost all of the world’s opal supply comes from Australia.

Topaz ”“ November’s Birthstone

Topaz Birthstone November

The November birthstone is topaz.  Topaz is a unique gemstone that comes in a huge variety of colors such as orange-yellow, colorless, light blue, pink, brown, and green.  Topaz has a hardness of 8 on the Mohs Hardness Scale and a glassy luster.  Igneous rocks are the most common type in which to find topaz gemstones.  Topaz is popular in jewelry because of its variety of colors.

Turquoise ”“ December’s Birthstone

Turquoise Birthstone December

The December birthstone is Turquoise, a blue-to-green mineral that is moderately hard and typically shows no specific crystal structure. Turquoise is a 6 on the Mohs Hardness Scale of 1 to 10, has a pale blue to white streak, and has a waxy luster. “Cryptocrystalline” is the term for minerals like turquoise where the crystals are too small to be seen.  Turquoise polishes to a beautiful shine making is a popular jewelry stone.

Fluorite – A Special Mineral of Many Forms and Colors

Fluorite is a unique mineral that forms in more colors than any other mineral on Earth. The most common colors are blue, red, purple, yellow, green, and white. In developing the Mohs Scale of Mineral Hardness, Friedrich Mohs chose fluorite as the example mineral for the hardness of four (4) on the scale.

Italian mineralogist Carlo Antonio Galeani Napione named the mineral in 1797. The name fluorite is derived from the Latin word “fluere” which means “to flow.” Fluorite was often used as a flux in iron smelting, which is believed to be the reason behind the name choice.

Fluorite often forms in low to high temperature hydrothermal veins inside rocks, especially those containing lead and zinc minerals. It is also present in some igneous and metamorphic rocks. Fluorite is often found with other minerals such as galena, calcite, quartz, sphalerite, and borite. Crystals of fluorite can form in cubes (squares) or octahedrons (diamond) shapes.

Common Properties of Fluorite

Formula: CaF2
Other Names: Fluorspar
Color:  Blue, Red, Purple, Yellow, Pink, Champagne, Brown, Green, or White
Hardness:  4 on Mohs Hardness Scale
Streak:  White
Luster:  Glassy to Dull
Density:  3.175 – 3.56 g/cm3
Crystal Shape: Cubes, Octahedrons, Dodecahedrons. Sometimes twinned crystals occur.
Fracture: Conchoidal to splintery
Cleavage:  Perfect Octahedral

Beautiful samples of fluorite exist in Germany, Italy, Switzerland, England, Mexico, Canada, and the United States. Fluorite is a common component in enamels, hydrofluoric acid, cooking utensils, telescopes, camera lenses, jewelry, and home décor items.

Fluorite Mineral

Fluorite Mineral Fun Facts

  • Fluorite crystals often grow together which is called “twinning.”
  • Fluorite crystals will fluoresce under ultraviolet light.
  • Jewelers sometimes facet fluorite crystals and use them as imitation diamonds although it is a much softer mineral making fluorite less ideal as a gemstone.
  • In addition to forming cubes and octahedrons, fluorite can occur in bands. One of the most famous forms of banded fluorite is called Blue John and found in Derbyshire, United Kingdom.
Fluorite Octahedrons

A Spectacular Rare Form of Fluorite

A rare type of striped, or banded, fluorite is the Blue John Fluorite found in the Blue John and Treak Cliff caverns in England. These caves are located in the Peak District in England near the town of Castleton, Derbyshire. The caverns formed within limestone strata that deposited in deep ocean waters millions of years ago. Layers of shale and some gritstone, a type of coarse sandstone common in this area, cover the limestone. The geology of the Blue John Cavern is fascinating and discussed further in a separate blog post.

Blue John Fluorite

This rare form of fluorite is so fascinating that it was the catalyst for an entire book series titled, the Crystal Cave Adventures. This book series starts when teenagers Emma and Brody fall through a time-traveling cave and land in 1775 England to steal a sample of the rare mineral for their friend, geologist Mr. M (aka Heath Matthews). Mr. M’s stunning rock and mineral collection is scheduled for national display when a thief breaks into his house and destroys all of his samples and maps. Emma and Brody vow to help rebuild the collection, later discovering the secret, magical time-traveling cave that Mr. M used to collect his unique samples. With one simple step, the adventure begins.

Blue John's Cavern

From the book, Blue John’s Cavern:

         “The Blue John Fluorite is one of the rarest forms of a common mineral. Its name comes from the French words ‘Bleu Jaune’ which translates into English as ‘Blue-yellow.’ The Blue John form of the mineral is located in a hillside near a large mountain called Mam Tor, which is just outside of the town of Castleton,” Mr. M said. He stopped walking and opened a long tube he was carrying and removed a street map. Mrs. M, Emma, and Brody held the map open as he pointed to a solid green area that showed the grassy hills of Derbyshire, England. Nestled in the rolling hills were the town of Castleton and a historic castle called Peveril. “The fluorite is found in two caves to the west. One cave is named Treak Cliff, and the other is called Blue John. Inside the caves, the fluorite runs in veins that are as thick as your fist. The fluorite always has bands of colors like blue and purple or white and yellow.”

         “Because of its beauty,” he said rolling the map and placing it back into the cylinder, “the Blue John Fluorite was largely mined and used to make jewelry, statues, and vases.” The group kept walking. Soon they were standing in front of a thick, woven mass of vines that looked like a door covering. “This is where you’ll find the Blue John Fluorite,” Mr. M said.

You can find out more about the first book in the Crystal Cave Adventures book series, Blue John’s Cavern, on our website.

Check out our website www.MiniMeGeology.com for great cubic fluorite samples and fluorite octahedrons. If you have questions about fluorite or any of our samples, contact us! We are happy to help you discover new, exciting samples for your collection.