Identifying the Rocks of Lake Michigan (Geode, Septarian, Agate, and More)
I Found a Rock on the Beach and Wondered
Looking west towards Chicago and Milwaukee from our Lake Michigan shorelines during a beautiful September sunset is a favorite pastime for us Michiganders.
Another favorite pastime for all citizens of the Great Lakes States is combing the beaches for interesting treasures. Beachcombing can be a very settling and spiritual experience. Beachgoers bring along a bucket or two in case they find cool fossils to collect or beautiful samples of driftwood—or perhaps colorful, sand-smoothed pieces of beach glass—but especially fascinating rocks.
I have enjoyed collecting all the above from my nearby Southwestern Michigan beaches and would like to share with you some of the rocks I have discovered washed up onto the shore. Some samples are very common, while the more unusual are found only in certain places. I will present beautiful rock photographs for your enjoyment and pass along fascinating information about each find!
Ah, where to begin . . . the above photo is an example of what I often find during the springtime due to winter's push of ice and snow that deposits rocks further up onto the shore. Later in the season, the waves move the rocks back into the water along the shoreline or the wind buries them with sand. So I find spring to be the best season for rock hunting. But I must mention: some Lake Michigan beaches have very few stones, while other pockets are loaded with them.
The photo above shows samples of basalt beach stones together with Septarian brown stones. Basalt is the most common stone (other than granite) found along the shoreline where I live in Southwestern Michigan. I was especially drawn to this assemblage lying on Pier Cove Beach by the way the late afternoon sun lit up the contrast between the warm and cool complimentary colors.
Note: One consistency you'll find with beach stones and rocks is their smoothness and rounded edges. That's due to the wind and waves pushing the stones against the sand, acting as a polisher. The smoothness is also an indication of how far a stone has traveled from the site of its original formation. The smooth rocks feel so good and healing in your hand!
This article includes various types of basalt, septarian brown stones, limestone, granite, gabbro, diorite, gneiss, schist, sandstone, siltstone, mudstone, geodes, chalcedony, and agate. If you don't see a match for the stone you've found here, you might find it here: Common Beach Stone Identification (Including Dolomite, Quartz, Serpentine, Syenite, and More.
Rocks are combinations of minerals, and minerals are combinations of chemical elements. Basalt is a fine-grained, dense volcanic rock, the original rock of Earth's crust. It covers more of Earth's surface than any other rock. It is formed from ancient molten rock that quickly cooled when it reaches the surface (called "extrusive type"). This is the reason for its fine-grained, heavy density before gas bubbles, crystallization, or foreign materials infiltrate.
Most extrusive igneous rocks in Michigan were formed from ancient, quiet lava flows which reached the surface through long cracks and crevices; also, from remnants of mountain peaks that have withered away. Just imagine, when you find a basalt rock on the beach, you're holding in your hand a billion-year-old chunk of Earth.
Pyroxine and Plagioclase Minerals
Basalt is composed of plagioclase and pyroxine minerals. If you're studying rocks, these terms will come up often.
- Pyroxine is a class of rock-forming silicate mineral, generally containing the chemical elements calcium, magnesium, iron, and occasionally the mineral olivine. In lesser quantities, basalt may also contain the mineral quartz, hornblende, nepheline, orthopyroxene, etc.
- The Plagioclase is a group of related feldspar minerals that essentially have the same formula but vary in their amounts of the chemical elements sodium and calcium.
Note: Many explanations are simplifications for us non-geologist laymen. Researching rocks can get pretty intense and involved.
What Color Is Basalt?
Basalt is usually grey to dark grey, but can rapidly weather to brown or rust-red due to oxidation of its iron rich minerals and further exhibit a wide range of shading due to regional geochemical processes.
Ophitic Basalt looks like a basalt rock that has been decorated with lighter-colored painted snowflakes. They come from tiny feldspar crystals that were in the lava. Because the crystals have eroded at different rates than the basalt, there is often a slight mottled texture to these stones. The sample above was a small boulder found on the beach and was quite heavy to carry in my arms!
Occasionally, we find these most-curious pitted stones on the beaches. After some research, I'm glad I finally understand how they got that way. They are called "Vesicular Basalt," which means basalt with textures, and if the deep pits (vesicles) cover more than half the surface of the rock, it's called scoria.
What causes the vesicles or pits in the rock?
Here's what happens: The basalt-making molten rock cools down quickly before gas bubbles from deep inside Earth's surface have the chance to make their way out. When the lava reaches the atmosphere, the bubbles inside can blow out, leaving spherical-pitted impressions.
Here's what can happen, yet later: The vesicles can fill in with other minerals and the fillings are called amygdules. The basalt is then referred to as amygdaloidal basalt. If the lava flow is in motion when the blowholes are being formed, the holes may be drawn out and elongated, as you can see in the sample above.
What is porphyry?
In various rock types (in this case, basalt), when you see large crystals of a mineral embedded within other finely ground minerals making up the mass, it's porphyry or porphyritic rock. You can tell the porphyry basalt apart from the above amygdaloidal basalt by the absence of empty pits. These are more rare to find on the beach. This sample of basalt has a greenish cast due to the inclusion of the mineral olivine.
Brown Septarian Stones
Septarian stones are typically brown due to the iron content. Composed mainly of mud and clay that formed on the ocean floor around 50 million years ago, at some point, perhaps from dehydration, the stones cracked. Later, calcite infiltrated the open veins via ground water, gradually filling them up.
This is one of those types of stones that is found only in certain areas of Southwestern Michigan and very few other places around the world. Locals call them "lightning stones" or "turtle stones" for obvious reasons.
The photo below is a good example of the cracking process. Sometimes the stones break completely apart and we find thousands of smoothed, broken-off sections on the beach.
There are many varieties of limestone lying on the beaches of the Great Lakes. I've provided information about a few, but briefly, here's a basic definition:
Limestone is a sedimentary rock composed mainly of the skeletal fragments of marine organisms such as coral, clams, and mollusks. Its major materials are the minerals calcite and aragonite, which are different crystal forms of the compound calcium carbonate. Marine animals grow their shells by extracting calcium carbonate from the water, which is fascinating to me.
Crinoidal limestone contains a significant amount of crinoid fossils. Crinoids are branching, stemmed, plant-like, mostly extinct organisms that lived 500 million years ago. They are animals that sift microorganisms from the ocean water.
With keen eyes, we can often find crinoid "sea lily" individual stems or broken-off cheerio-shaped pieces of the remains of these marine creatures. Native Americans wove necklaces with them.
Here, I provide several samples of fossiliferous limestone to demonstrate how abundant they are on our beaches, especially in Southwest Michigan. Fossiliferous limestone contains obvious and abundant fossils such as the shells of mollusks, clams, crinoids, and other invertebrate organisms. Like other limestone, fossiliferous limestone is composed of the mineral calcite. It can be white, pink, red, reddish brown, gray, and even black, depending on the mineral makeup. We find many of them redddish-brown-colored due to an infusion of iron.
Tuffa Limestone is a porous limestone that forms from the precipitation of calcium carbonate, often at a hot spring or along the shoreline of a lake where waters are saturated with the chemical compound.
Compact Limestone is composed primarily of tightly packed calcium calcite derived from the remains of marine organisms. It can vary in color from white, yellowish, pink, red, gray, or even black, depending on the presence of other minerals. It has a very fine texture and is denser than other types of limestone. The first sample shown above is a large piece with rounded edges but has been flattened, hence the name "shingle" for flat stones found on beaches.
Dolostone or Dolomite?
I just love finding the perfectly round or egg-shaped samples of smooth, white limestone rocks!
Perhaps the first sample I provided is dolostone or dolomite (not to be confused with the mineral). It could very well be a dolomite rock which is usually quite similar to limestone, and the two are often indistinguishable in the field. Geologists carry small bottles of diluted hydrochloric acid to test carbonate rocks.
The most common type of dolomite rock is a former limestone that was dolomitized.
What is dolomitization?
Dolomitization means that calcium carbonate (the minerals aragonite or calcite which make up limestone) were replaced by calcium magnesium carbonate (mineral dolomite) through the action of magnesium-bearing water percolating the limestone or limy mud. Dolomite rock is another very common sedimentary rock. Older carbonate rocks formed before the Mesozoic tend to be dolomite, whereas younger carbonates are predominantly various limestone.
It's thrilling to find these round, bird-egg-shaped granite stones on the beach. With their varied colors and patterns, they create beautiful works of art. Granite is another type of rock we find quite often on our Lake Michigan beaches in the form of pebbles, cobblestones, and boulders.
Granite makes up 70–80% of Earth's crust. It's an igneous rock that cooled slowly deep within the Earth, allowing the slow process of the crystallization of molten rock (intrusive type). The coarse-grained minerals are seen easily with the naked eye. Colors vary from red, pink, gray, to white with black grains, depending on the amount and mix of minerals.
What gives granite its color?
- Quartz - typically milky white in color
- Plagioclase Feldspar - typically off-white
- Alkali or Potassium Feldspar - typically salmon-pink
- Biotite Mica- typically black or dark brown
- Muscovite Mica- typically metallic gold or yellow
- Amphibole Hornblende- typically black or dark green
Can you guess their mineral content based on their color?
Above, two samples demonstrate the variances in granite's color, depending on mineral content. Can you guess their mineral content based on their color?
Although granite underlies much of the Earth's surface, it doesn't often rise up to where we can see it. The Canadian Shield is an enormous granite formation covering most of the country. It is the nearest place to Michigan where granite is found above the crust. So how did it get to Michigan's shores? You guessed it . . . glaciers from past ice ages scraped up the material and brought it south.
Porphyry or porphyritic rock is a finer-grained igneous rock containing large, jagged crystals of feldspar. The porphyritic crystals are generally white, pink, or orange. Porphyry rock is mostly made up of a basalt base but sometimes granite with jagged, rectangular crystals.
With granite, it’s harder to identify the porphyry because of the already-coarse grain, but look for stubby, square, or hexagonal crystals that are larger than the other grains within the granite rock. You can clearly see this in the sample I have provided above, which was found on our Lake Michigan beach.
Here's how it happens: As the feldspar begins to crystalize, the process is disturbed when the molten rock is quickly erupted, freezing the well-formed feldspar crystals in place while the rest of the rock quickly cools and fills in around them.
Gabbro is igneous rock which cooled slowly (intrusive) deep below the Earth's surface, which caused the minerals to crystallize. It's sometimes called “black granite” for its similar, coarse-grain appearance, but a large proportion of iron-bearing minerals make gabbro heavier and usually darker-colored.
Gabbro can also be gray and dark green. You may see fewer light-colored mineral grains. Unlike many other igneous rocks, gabbro usually contains very little quartz, although the sample I collected has a quartz vein running all the way around it.
Gabbro has the same mineral composition as basalt (olivine and pyroxene, with smaller amounts of feldspar and mica). But whether basalt or gabbro forms depends upon the cooling rate of the magma, not its composition. While gabbro is coarse-grained, which cooled slowly (intrusive), basalt is fine grained because it cooled quickly (extrusive).
Diorite is another of several types of coarse-grain igneous stones that can easily be confused with granite. Diorite's chemical composition is intermediate between gabbro and granite.
How to tell the difference between diorite, granite, and gabbro?
The best way to tell diorite from granite is that the latter has that salt-and-pepper appearance. To tell diorite from gabbro, look for gabbro's darker color. If you have in your hand a granite-looking rock with obvious pink feldspar and more than 20% quartz, you probably have granite, not diorite or gabbro. Phew . . .
Diorite is composed with an almost-equal mixture of the light-colored mineral, sodium-rich plagioclase (a different type of feldspar mineral), and dark-colored minerals such as amphibole, hornblende, or biotite mica.
Did you every wonder how some rocks have bands or stripes? They are some of the most attractive stones, like gneiss for instance, which I only occasionally find on the beach.
Gneiss (pronounced "nice") usually forms at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallize, enlarge, flatten, and reorganize into parallel bands under intense heat and pressure which make the rock and its minerals more stable. While the chemical composition of the rock may not have changed, its physical structure will look completely different from the original parent rock.
The bands in gneiss are often broken, can be folded (foliated), and can be different widths. Individual bands are usually 1-10 mm in thickness. Layers larger than that imply that partial melting or the introduction of new material probably took place. Such rocks are called "migmatites." Hence, my boulder sample above would be termed “migmatized gneiss.” It is not well understood how the segregation takes place.
The granular light-colored minerals in gneiss are calcium-, sodium-, and potassium-rich minerals such as quartz, and also various types of feldspar. The dark colored layers consist of iron-magnesium-rich minerals including biotite, chlorite, garnet, graphite, and hornblende.
The texture is medium to coarse—coarser-grained than schist but, as with the other rock types, the gneiss we find on our beaches has been ground down until it's somewhat smooth.
What is the difference between gneiss and granite?
- Granite is an igneous rock, whereas gneiss is formed after metamorphosis of granite.
- Most—but not all—gneiss is obtained from granite. There is also diorite gneiss, biotite gneiss, garnet gneiss, and others.
- The mineral composition of granite and gneiss are the same. However, the transformation of granite due to high pressure and temperature leads to the formation of gneiss.
Schist is medium-grade metamorphic rock formed by the metamorphosis of mudstone and shale or some type of igneous rock such as slate. As a result of high temperatures and pressures, the coarser mica minerals (biotite, chlorite, muscovite) form larger crystals. These larger crystals reflect light so that schist often has a high luster. Due to its extreme formation conditions, schist often shows complex folding patterns. It also shows a tendency to split into sheets. The mica plates are arranged roughly parallel to each other, which is why the rock shows this tendency.
There are many varieties of schist, and they are named for the dominant mineral comprising the rock, e.g. mica schist, green schist (green because of high chlorite content), garnet schist, and so on.
I find these somewhat regularly on the shoreline. The lustrous, shiny samples are much less common, of which I've found only one.
Sandstone is a sedimentary rock that forms when small quartz sand grains cement together under high pressure while silica, calcium carbonate (calcite), or quartz precipitates and acts like a glue around the grains. These minerals are deposited in the spaces between the sand grains by water. Over the course of thousands or even millions of years, the minerals fill up all of the spaces. You can see the tiny particles in the rock as if you were holding sand in your hand. When you're at the beach, try examining the sand very closely for all the tiny quartz crystals and different colors of other minerals contained in it, including feldspars, micas, calcite, and clays.
Depending on the minerals, sandstone can be white, yellow, pink, and almost any color, depending on the impurities within the minerals. For example, red sandstone results from iron oxide in the rock and often causes bands of color. Sandstone rocks form in rivers, deserts, oceans, or lakes.
Jacobsville Redstone Sandstone
Jacobsville Sandstone or Redstone is generally red due to the presence of highly oxidized iron cement which binds together the grains of quartz. It's typically mottled with various pinks, whites, and browns, exhibiting either many streaks or spherical spots caused by leaching and bleaching. It forms a wide belt through Northern and Upper Michigan and was quarried rather extensively at one time for use as building material to build the cities of Northern Michigan and elsewhere in the Great Lakes region. As with many stones formed elsewhere in Michigan, the big lake brings them southward to where I find them in lesser amounts.
Estimates for the age of the Jacobsville Formation Range is that it formed in the late Mesoproterozoic Era about 1.05 billion years ago until the Middle Cambrian Period.
After some stubborn digging around, I finally believe I understand the difference between sandstone, siltstone, mudstone, claystone, and shale. They all fall under “clastic” sedimentary rocks formed by weathering breakdown of rocks into pebbles, into sand, then silt, and so forth (you get the picture) from exposure to wind, ice and water. At each step the particles become smaller with shale having the finest grain.
All the clastic sedimentary rocks mentioned above are cemented very much the same way in which sandstone is pressed together. Silica, calcite, and iron oxides are the most common cementing minerals for siltstone. These minerals are deposited in the spaces between the silt grains by water. Over the course of thousands or millions of years, the minerals fill up all of the spaces resulting in solid rock.
Silt accumulates in sedimentary basins throughout the world. It occurs where current, wave, or wind energy cause sand and mud to accumulate.
Siltstone is very similar in appearance to sandstone, but with a much finer texture. It has a gritty texture to it and is more difficult to distinguish the mineral particles. When handling siltstone, a residue the color of the stone can rub off on you hand.
Siltstone is usually gray, brown, or reddish brown. It can also be white, yellow, green, red, purple, orange, black, and other colors. The colors are a response to the composition of the grains, the composition of the cement, or stains from subsurface waters.
Mudstone or Claystone
I already described how mudstones and claystones are clastic sedimentary rocks formed similar in the way as sandstone and siltstone. But I will mention that we especially find the brown mudstones on the beach. They are the same type of stone that form the septarian brown stones. Both the mudstones and claystones wipe off a residue when you're handling them from their fine-grained texture.
The last stone in the chain of the clastic stones for the finest-ground-down grains is slate, which we find very little of. This could be because slate breaks apart at parallel stratifications and, perhaps because of the extreme ice, wind, and wave action of Lake Michigan, they get demolished, but I can only guess.
Geodes are one of the less-common finds on beaches, but it's very exciting when you do find one. They begin their formation as hollow rocks. As a volcanic rock, geodes start out as bubbles, but can also form in areas other than volcanoes. In sedimentary rocks, geodes can start out as animal burrows, tree roots, or mud deposits which, over time, form the hollow cavity within the rock while the outer edges harden and form a sphere. Mineral-rich ground water infiltrates the cavity and after many years, the minerals crystallize in various colors depending on the mineral content such as quartz and amethyst.
Chalcedony and Agate
Without being totally certain, I believe these pretty little stones are one type or another of the gemstone chalcedony. They are only as big as a penny and have a smooth, waxy texture. In order to spot these on the beach in Southwestern Michigan, you have to look very closely along the shoreline where gravel is abundant, but I have found quite a few and they're usually tiny like these samples. Michigan's northern regions and upper peninsula are excellent places for finding agates.
Chalcedony and Agates Explained . . .
Rock and minerals can be very complicated but fascinating to study. For a bit of geochemistry about chalcedony and agates, it only makes sense to begin with the microchrystalline quartz, chalcedony. Chalcedony forms where water is rich in dissolved silica and flows through weathering rock. When the solution is highly concentrated, a silica gel can form on the walls of the rock cavities. That gel will slowly crystallize to form microcrystalline quartz (very small crystals of quartz) or, in other words, chalcedony. So why begin with chalcedony? Because agate and jasper are both varieties of chalcedony. They are considered gemstones.
Chalcedony can be banded, have plume (fluffy inclusion), or have branching patterns and a delicately mottled surface of leafy green, honey brown, and creamy white. They might also have mossy and other colorful structures within. Chalcedony is often blue but can be almost any color. It's always translucent, never opaque or transparent. It feels waxy, greasy, or silky.
The Formation of Agate and Jasper
Both agate and jasper form somewhat differently from chalcedony, giving them their unique properties. Many agates form in areas of volcanic activity and crystallize slowly in the cavities of igneous rock or limestone. Jasper forms when fine materials are cemented by silica to a solid mass. These fine particles give jasper its color and opacity.
How can you tell agate from jasper?
Agate is generally translucent to semitransparent and most often banded. Observing bands in a specimen of chalcedony is a very good clue that you have an agate. However, some agates do not have obvious bands. These are more rare and often translucent agates with branching-out, mossy inclusions. Typically, an agate is the size of a golf ball and feels heavier than it looks due to its density. It also has a waxy feel to it.
Jasper is opaque because it contains enough non-chalcedony material to interfere with the passage of light. So the real difference between jasper and agate is the amount of impurities and foreign material contained in a specimen. Jasper is almost always multicolored, with unique color patterns and habits.
Note: Chert—a sedimentary material that forms in a similar way as jasper, biochemically from silicon-based marine skeletons of plankton—is yet another form of chalcedony sometimes called jasper.
More forms of chalcedony distinguished by color include:
- Aventurine (opaque green)
- Bloodstone (dark green, red speckles)
- Carnelian (red to amber; translucent)
- Chrysoprase (apple green)
- Onyx (solid black or white-banded black)
- Sard and Sardonyx (reddish-brown banded; transparent, translucent)
- Tiger's Eye (gold, banded, glistening sheen)
Beautiful Photos of Beach Stones in Lake Michigan's Pier Cove Creek
I leave you with beautiful photos of Pier Cove Creek, which flows into Lake Michigan in Southwestern Michigan. During the spring meltdown, the creek rushes into the lake with more power than usual, exposing the many buried, rounded stones and pebbles. Listening to the trickling water and watching the tiny sunlit ripples on the water is a delightful experience with Mother Nature herself.
Which rocks have you collected the most?
Questions & Answers
We found a shiny silver nugget with our metal detector, looks like it's possibly platinum ore. It was buried on the shore of the bay in northeast wi & so. How do we know what we have?
If you've already performed a search, you likely are already on the right track. Testing whether it's magnetic narrows the search, streaking the nugget on unglazed porcelain for color also narrows the identification. Scratch testing for hardness also helps!Helpful 15
I found a dark and light gray striped stone in Milwaukee on LM this week. What is it? Looks like a tiger stripe.
Likely metamorphic basalt or slate.
I have two very lightweight rocks that are black, opaque and, although they aren't shiny, they are shinier than most Lake Michigan stones and rocks I've picked up. For this area, I can't find what type they are. Can you help?
It sounds like they could possibly be pumice.Helpful 11
I found a white round rock on a beach. It is freckled with what appears to be silver glitter. It's quite porous. What is it?
Without actually seeing it, could be granite or a type of granite called diorite with the silver glitter being mica.Helpful 9
I found a smooth rock that looks like quartz but has dark purple on it. Could it be an amethyst?
Very possible, amethyst is a type of quartzHelpful 8