Geological State Symbols Across America  Geology of the National Parks Through Pictures

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Arkansas State Geological Symbols
Year Est.
State Rock
State Mineral
Quartz Crystal
State Gemstone
State Dinosaur
Arkansaurus fridayi


State Rock: Bauxite


An example of bauxite from Little Rock, Arkansas.

Photo courtesy of

Bauxite is a sedimentary rock, which is usually produced when most of the minerals are leached from the soil leaving high levels of alumina behind in wet tropical or subtropical environments. Bauxite is often thought of as an aluminum mineral (that is even how it was present in several intro geology labs I was in charge of) however it isn't just one mineral; bauxite is primarily composed of gibbsite (Al(OH)3), boehmite (AlO(OH)), and diaspore (AlO(OH)). All of these minerals are extremely soft, occurring between 1 and 3 on Moh's Hardness Scale, meaning bauxite itself is a soft rock. The formation of bauxite produces a pissolitic texture, which resembles little balls glued together (as seen in the picture to the left). The majority of the aluminum in the world comes from bauxite ores. To extract the aluminum from the bauxite, the bauxite is crushed into a powder and the aluminum is leached out via several chemical procedures. The obvious use of aluminum is as a metal, but it can also be used for abrasives (one of the by-products of the leaching process has a hardness on Moh's scale of 9), in cements, and as proppants (high density spherical grains used by the oil and gas industry in fracking). Currently, the United States is not even in the top ten for bauxite producers and it is only found in a handful of localities in the United States (Arkansas, Georgia, and Alabama).

Bauxite location map

Mineral resources of Arkansas, highlighting the Bauxite deposits, which are the light pink areas. Map courtesy of the Arkansas Geological Survey.


Arkansas bauxite was formed from richly alkaline igneous intrusive rocks that intruded into the Paleozoic age sedimentary rocks of the central region of Arkansas. The weathering of these igneous intrusive rocks during the Eocene provided the material to form the bauxite deposits. The igneous rock intrusion is a complex series that includes bodies of nepheline syenite and porphyritic alkalic syenite, as well as dikes and border phases of related rocks. These rock strike towards the northeast of Arkansas and dip gently towards the southeast. The bauxite is found within two regions of Arkansas, covering 275 square miles, located within Pulaski and Saline Counties. These are mapped on Mineral Resources map (to the right). Bauxite deposits are highlighted by the pink areas covering the two counties. In Arkansas, bauxite production started in 1899, and increased until 1923 when Arkansas produced half of the world's supply at 500,000 tons that year. The peak of production was in 1943 when 6,000,000 long tons were produced, but it has had a steady decline ever since. In 1982, mining of bauxite was ceased for aluminum metal with the current bauxite mining being used primarily for the production of proppants.



State Mineral: Quartz Crystal

Quartz Crystal

Quartz Crystal from Arkansas. Image courtesy of

Quartz is one of the most common minerals on Earth, primarily due to its simple structure and chemical formula, SiO2. Quartz also has an extremely high hardness, 7 on Mohs hardness scale, meaning that it doesn't scratch very easily and therefore does not break down easily. As the rocks on Earth are slowly eroded over time, most of the other minerals will break down into clay while quartz grains will generally just gets smaller and smaller. The result is that most beach sand is composed of quartz that has a slight hematite (rust) stain to it to give the sand grains their slight yellowish color. Although quartz is a simple mineral, it can come in a variety of colors depending on what type of impurities are present in the crystal structure; pure quartz crystal is clear, milky quartz is white, smoky quartz is grey, amethyst is purple quartz, citrine is yellow quartz, rose quartz is pink, as well as some other colors and varieties. Quartz does not have any cleavage, meaning that when it breaks it doesn't form along perfect surfaces. Instead as the quartz crystals grow, individual mineral molecules of quartz are added to the outside of the crystal from water rich in dissolved SiO2 or mineral melt (liquid rock like lava or magma).


Large Quartz Crystal

Worlds largest quartz crystal, mined at Old Coleman Mine in Jessieville, Arkansas. Image courtesy of

Although the mineral quartz is the most common mineral on the surface of the Earth, quartz crystals are a little rarer. The deep running hydrothermal waters in Arkansas, which had made the town of Hot Springs famous, have produced some of the finest varieties of quartz crystals on the planet. The Ouachita Mountains, home to the hot springs, has been known for centuries by Native Americans for their beautiful quartz crystals that were considered to have spiritual and sacred significance. The Ouachita Mountains began to be formed 600 million years ago, when a local rift basin started to open up. This rift basin accumulated sediment that eventually formed into sandstone and shale. These deposits were subsequently uplifted producing the mountain complex. Hydrothermal fluids started circulating through these rocks approximately 245 million years ago producing silica rich fluids that precipitated the quartz veins within cracks in these rocks. Native American artifacts from these rock crystals date back to 9,000 BC., with many of the early Native American arrow tips being fashioned from the quartz crystals themselves. In the modern days, these quartz crystals are considered to be some of the most beautiful quartz crystals in the world. People travel from all over to go to Mount Ida and Hot Springs to the many "dig your own quartz crystal" mines. The town Mount Ida even proclaims itself the "Quartz Crystal Capitol of the World". Currently, there has been an estimated 1,200 tons of quartz crystal ever mined from this region, totalling approximated 5% of the estimated total quartz crystal reserves. There is still plenty of quartz crystals left to be mined, perhaps finding one larger than the current world's largest quartz crystal, which was mined from the Old Coleman Mine in Jessieville, Arkansas (pictured left).

State Gemstone: Diamond

Cullinan Diamond

The Cullinan Diamond, which was polished into nine large diamonds and many more smaller diamonds. Image courtesy of The Cape Town Diamond Museum.

One of the most famous gemstones, the diamond, is also one of the hardest minerals on earth (it is actually the third hardest after two extremely rare minerals called Wurtzite Boron Nitride and Lonsdaleite ( A diamond is made up entirely of carbon (like its mineral cousin graphite), where the arrangement of the carbon atoms and the strength of the bonds are what give the two minerals completely different properties. Diamonds are measured in carats, where a carat is equal to 200 milligrams. Diamonds range in size from the microscopic diamond dust to the largest diamond ever discovered, the Cullinan Diamond, weighing in at 3106 carats, or ~1.4 pounds. The Cullinan Diamond (pictured right) was eventually cut into over 100 separate stones, with two of the largest stones forming part of the Crown Jewels. Diamonds are most often found in structures called kimberlites or lamproites. Kimberlites are magmatic rocks that are formed deep within the Earth. The high pressure converts the carbon into diamonds and the structures make their way to the surface as buoyant globs of rock. Due to being formed at such high pressure, diamonds are inherently unstable on the Earth's surface, however they degrade at such a slow rate that it isn't much of an issue in jewelry. Many diamonds today are found in these isolated structures, however not many kimberlites or lamproites are known to exist within the United States, with only two knowingly existing.



Uncle Sam Diamond

The Uncle Sam Diamond, which was the largest diamond every found in the United States. It was found in the location which eventually became Crater of Diamonds State Park, Arkansas. Image courtesy of

Arkansas Diamonds

Some of the diamonds found at Crater of Diamonds State Park in Arkansas. Image courtesy of Arkansas State Parks.

The main locality for finding diamonds in the US is a lamproite located in Crater of Diamonds State Park, Arkansas. Here visitors are allowed to mine for diamonds and keep what they find (as pictured to the left); the only such diamond mine on Earth. It was originally assumed that the diamonds in Arkansas were formed from a kimberlite, however recent research has shown that lamproites, which differ from kimberlites in how they are formed and in their chemistry, more closely align with the deposits in Arkansas. Generally, the diamonds found within Arkansas have a tendency to be smaller in size than their counterparts in Africa. However, there have been some significant finds including the the largest diamond ever found in North America, called "The Uncle Sam". The Uncle Same was discovered in 1924 and weighed 40.42 carats before it was cut (pictured right).


State Dinosaur: Arkansaurus fridayi

Arkansaurus Drawing Reconstruction

Arkansaurus fridayi reconstructed illustration by Brian Engh


Arkansaurus cast

Cast of the reconstructed Arkansaurus fridayi material.

Photo courtesy of ReBecca Hunt-Foster.


Besides fragmentary bits of bones and some dinosaur tracks, Arkansaurus fridayi is the only known dinosaur fossil recovered from the state of Arkansas to date. The fossil was initially discovered by Joe B. Friday on his land in 1972 during an earthmoving project. The fossil comprised of bones from the right foot of the animal. They were discovered within the Early Cretaceous Trinity Group (Albian-Aptian, ~113 million years ago) near Locksburg, Arkansas. These bones were initially described back in 1973, however not much else was done with the fossil material until 2018 when they were redescribed by Hunt and Quinn as a new species of ornithomimosaur (the kind of dinosaurs you saw running around in flocks on Jurassic Park). The remains of Arkansaurus were compared to other known dinosaurs and determined to be an early relative of the Late Cretaceous ornithomimids Ornithomimus and Struthiomimus. Ornithomimids are usually identified by their resemblance to modern day ostriches (hence the name "ornithomimid" means "ostrich mimic"), and the bones of Arkansaurus were similar enough to the known ornithomimids to declare it as an early ancestor of those species. The genus name Arkansaurus means "Arkansas' reptile" and the species name fridayi was in honor of Joe B. Friday, who initially discovered the remains.



ReBecca K. Hunt & James H. Quinn (2018): A new ornithomimosaur from the Lower Cretaceous Trinity Group of Arkansas, Journal of Vertebrate Paleontology

Geology of Arkansas's National Parks

Through Pictures

(at least the one's I have been to)

Hot Springs National Park

Hot Springs National Park

Visited in 2003 (before I was focusing on the geology in pictures)


Hot Springs National Park

The water within Hot Springs National Park is unique in that it has traveled a very long way down into the ground surface. These waters have been heated up by geothermal activity (the heat inside the Earth) as opposed to the volcanic activity that you would see at springs in areas like Yellowstone. Here is Bathhouse Row, where the bathhouses are lined up along the areas where the natural springs come to the surface. 


Hot Springs National Park                      Hot Springs National Park

Since the water has been heated by geothermal activity, none of the nasty chemicals associated with volcanic hot springs are present in these waters. That makes it so these waters are able to not only be bathed in, such as at the bathhouse to the left, but also you can drink the water, such as at one of the free hot spring fountains on the right.


Hot Springs National Park

Although, many of the bathhouses obscure the natural emergence points of the hot springs, some of the natural springs can still be seen along the area, such as this spring adjacent to Bathhouse Row.