Kathi writes about fossils and other earthly subjects, plus the natural fauna of Michigan, features in her community, poetry, and more.
I Found a Rock on the Beach and Wondered
This article shines a light upon the beautiful beach stones we often find while combing the beaches of Lake Michigan. It is meant to help with rock identification, and also offers interesting facts about the rocks, beautiful photos about some of our common and more unusual beach stone discoveries, and curious information as to how some of our beloved beach stones were formed.
This article features various forms of syenite, rhyolite, pumice, dolomite, quartz, quartz veining, wishing stones, heart stones, quartzite, Presque Island serpentinite, diabase, pegmatite, conglomerate and banded metamorphic rocks, in that order!
Note: As with all beach stones, they are polished smooth from the sand, wind, and wave action by the fresh waters of our inland ocean, Lake Michigan.
You'll also find forms of basalt, septarian brownstones, limestone, granite, gabbro, diorite, gneiss, schist, sandstone, silt stone, mudstone, geodes, chalcedony, and agate. For those types, I have written another article, Identifying the Rocks of Lake Michigan (Geode, Septarian, Agate, and More).
Attractive, colorful syenite is a medium- to coarse-grained igneous rock related to granite which solidifies slowly in Earth's crust in a similar manner. Whereas quartz is an important mineral in granite, it's lacking in syenite.
Careful examination will show that syenite is composed of long prisms of the dark mineral hornblende (rather than the scaly biotite mica) and feldspar, which is the chief component of the rock.
The pink coloring of syenite is due to the presence of alkali feldspar which predominates in syenite, but this rock type is found in a wide variety of colors depending on the minerals contained within. They can include augite syenite, hornblende syenite, mica syenite, and nepheline syenite.
I just love finding unique rocks along the beach like these pink and pinkish-orange polka-dot syenite rocks! It's gratifying to know a thing or two about all rocks, but particularly the less common!
Syenite is not a common rock in Michigan, comparatively. They are occasionally substituted for granite as building stones.
Rhyolite is a felsic (silica-rich) volcanic igneous rock with the same mineral content as granite, only unlike granite, it cools fast from the molten rock near or over the surface of Earth's crust (extrusive type). When these magmas erupt, a rock with two grain sizes typically forms. The larger crystals that form just beneath the surface cool at a slower pace and are known as phenocrysts. The smaller, undetectable crystals that form at or above the surface cool quicker and are known as ground mass with a micro-crystalline matrix.
Note: You can see the two grain sizes more clearly in the samples of rhyolite shown below.
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Porphorytic Rhyolite. In order to understand this rock, we must know what "porphorytic" or "porphyry" mean! These are igneous rocks with one mineral (aka the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the ground mass).
Note: In the sample pictured above, the phenocrysts are quite a bit larger, in other words, porphorytic.
Michigan's Igneous Rock
The igneous volcanic rock of Michigan is very old. It slowly seeped through cracks and crevices in the Earth's crust during the molten state. If you hold a sample of rhyolite in your hand, you're holding something that likely formed a billion years ago or more . . . amazing.
Rhyolite rocks are often colored red to brown, pinkish to gray, with fine grain, sometimes exhibiting banding caused by molten rock that flows as it cools and hardens.
Rhyolite usually forms in continental volcanic eruptions and is rarely produced at oceanic eruptions. Rhyolites are known from all parts of the Earth and from all geologic ages.
Rhyolite Pumice. Due to its high silica content, rhyolite lava is very viscous . . . it flows slowly, like toothpaste squeezed from a tube, and tends to pile up to form lava domes. The thick viscosity traps gas bubbles and if rhyolite magma is gas-rich, it can erupt explosively, forming a frothy solidified magma called pumice (a very lightweight, light-colored, vesicular (pitted) form of rhyolite) which includes ash deposits.
Eruptions of Granite Magma
Eruptions of granite magma can produce rhyolite, pumice, obsidian, or tuff. These rocks have similar compositions but different cooling conditions.
- Explosive eruptions produce tuff or pumice.
- Effusive (slow) eruptions produce rhyolite or obsidian if the lava cools rapidly.
These different rock types can all be found in the products of a single eruption.
There is considerable confusion about the name of this rock. The problem is that dolomite is both a mineral and a rock type. Dolomite rock is a sedimentary rock derived from limestone with a high percentage of the mineral dolomite. The two types are often indistinguishable in the field so geologists usually carry diluted hydrochloric acid to test the rocks. Limestone is strongly effervescent in acid, while dolomite reacts very weakly.
As with limestone, dolomite rock originates in warm, shallow marine environments where calcium carbonate accumulates from shell, algae, or coral fragments. It is widespread in the Cambrian Period throughout the world. Limestone and dolomite rock also share the same color ranges: white to light gray, yellowish, green, pinkish, purplish, and even black are possible. Like limestone, dolomite rock can also exhibit fossils, but not as commonly.
I find boulder-sized dolomite at the beach in a variety of gorgeous colors. Arranged around my garden, they make an attractive addition while also serving as a border to contain soil around plantings! Sometimes I combine them with other pretty rocks for a special feature.
How does dolomite form?
Dolomite stone forms in several ways, but the main method is from a former limestone that was precipitated by calcium "magnesium" carbonate (mineral dolomite) through the action of magnesium-bearing water percolating the limestone or limy mud and replacing the limestone calcium carbonate minerals of aragonite and/or calcite.
Dolomite vs. Limestone
Again, when comparing dolomite to limestone, I find my beach samples to be more colorful. Also, dolomite rates higher on the Mohs hardness scale and you can feel the hardness in your hands.
Did you know quartz is the single most abundant mineral on the planet? Quartz is made up of the elements silicon and oxygen, otherwise known as silica. Quartz can form large, six-sided crystals over rocks or can be found within rock cavities such as granite, yet it also can fill rock vesicles (gas bubbles) during the cooling process of molten rock.
It can be found in a wide range of sizes such as masses larger than a basketball or crystal points smaller than a pea. Several varieties of quartz are microcrystalline (too small to be seen with the naked eye). These include agate, jasper, chert and chalcedony. Other quartz varieties are named for their different colors caused by impurities permeating during the crystallization process. For example, amethyst contains impurities of iron and aluminum, smokey quartz is colored by aluminum, and red quartz is iron-stained.
Milky quartz is the most common variety of crystalline quartz. The cloudiness of milky quartz comes from microscopic inclusions of fluids, gas, or both that have been encased in the crystal from the time the crystal first grew. The inclusions have spoiled the crystal for optical purposes and for the use in jewelry-making gemstones. But I still find them beautiful and feel excited when I stumble across one at the beach!
Holding the milky quartz up to the sun, the light can be seen through the translucence of this stone! Of course, we find these in their rounded, polished form due to the weathering action of the big lake!
The quartz veining in these samples certainly elicits curiosity. One can't help but marvel and wonder how. There are a few methods, but the simplest type of a quartz vein is caused by the filling of a crack in a rock. The crack might have formed during folding of rock in mountain-building processes, or by shattering during tectonic events, or by a decrease in pressure during the uplift of rock. Yet another way may be because a rock has cooled down and shrank. After the cracks are exposed, hot brines migrate through the cracks and crevices in the host rock, depositing various minerals which may or may not crystallize.
A "wishing stone" is nothing more than a stone with quartz veining. This occurs in various host rocks such as basalt, as in the case above! But in order to be a true "wishing stone," it can have only a single vein which circles entirely around the stone without any breaks. If you're lucky enough to find one, close your eyes, make a wish, then throw the stone into the water as far as you can and your dream wish will come true. The wishing stone in the photo above is shaped like a heart which makes it even more special!
Speaking of heart stones, these are especially favored by many beachcombers. Many have given or received heart-shaped stone as gifts, and these carry a special place in the receiver's heart! I like to think of them as heartwarming messages or hellos from loved ones who have passed!
Can you guess the various rock types of these heart stones based on the information I have already provided?
Quartzite is a metamorphic rock composed almost entirely from sandstone. The sandstone is altered by heat, immense pressure, and chemical activity. These conditions recrystallize the sand grains and the silica cement that bind them. The result is a network of interlocking quartz grains of incredible strength. Because it is so hard and dense, quartzite has not been quarried as extensively as other stones such as limestone, sandstone, and granite.
Quartzite ranks high on the Mohs hardness scale as one of the most physically durable and chemically resistant rocks found on Earth. When mountain ranges are worn down by weather and erosion, less-resistant rocks are destroyed, but the quartzite remains. Quartzite is also a poor soil-former. Unlike feldspars which break down to form clay minerals, the weathering debris of quartzite is quartz.
In identifying the rock, geologists have learned to strike quartzite with a rock hammer only when necessary and to tap softly to avoid possible sparks or sharp pieces of rock flying at high velocity due to the way the rock breaks apart.
What does quartzite look like?
Quartzite can be a very attractive stone when it is colored by inclusions. It is usually white to gray in color, but some rocks are stained by iron and can be pink or red as with these Lake Michigan samples. Other impurities from minerals carried by groundwater can impart hues of purple, green, blue, yellow, orange, or brown, which pretty much covers the color spectrum.
Presque Isle Beach Stones (Serpentinite)
These heavily veined cobble beach stones came from Presque Isle Park in Marquette, Michigan of Upper Michigan bordering Lake Superior. One source I found states it is serpentinized peridotite (serpentinite rock) of the Mona Formation, Archean in age - 2.6 billion years. Most locals simply call them Presque Isle Stones.
For those interested in this unusual rock's formation, here is a simplification for us non-geo types.
To begin,peridotite is a dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene with lesser amounts of chromite, plagioclase, and amphibole varying its compositions. Peridotite is ultramafic (a rock that contains less than 45% silica). It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron.
Peridotite is the dominant rock of the upper part of the Earth's mantle either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle.
How does serpentinite form?
Serpentinization is a process whereby rock (usually ultramafic as with peridotite) is changed, with the addition of water, heat, and pressure into the crystal structure of the minerals found within the rock. Serpentinization of peridotite into serpentinite (the metamorphic equivalent) is a common example of this process.
The unusual veining is more of a mystery because of its intricacy, but I have already explained how this process may occur as with the quartz veining explanation above. Anyway, we can all agree, it's complicated!
Diabase (dolerite is the older term) is a dark rock that may have light-colored, lath-shaped (flat, elongated) grains. Like basalt, rhyolite, and gabbro, diabase is a Michigan volcanic rock, only less commonly found along the shoreline as a beach worn cobbler or boulder stone.
It is equivalent to gabbro and basalt in composition, but between them in texture. The term “microgabbro” is sometimes used to refer to such rocks, but they cooled closer to Earth's surface, hardening much faster, and therefore have far fewer visible crystals than gabbro. They are classed as a separate rock because of the peculiar lath-shaped crystals of "plagioclase" lime-feldspar minerals (mostly labradorite) in a ground mass of the "pyroxene" mineral, augite.
To review plagioclase minerals; any member of the series of abundant feldspar minerals usually occurring as light-colored, glassy, transparent to translucent, brittle crystals.
To review pyroxene minerals; any member of a large class of rock-forming silicate minerals, usually dark-colored, generally containing calcium, magnesium, and iron and typically occurring as prismatic crystals.
Diabase minerals of lesser importance are magnetite, olivine, ilmenite, hornblende, biotite, and chlorite, among others.
Note: Specimens with few visible crystals can easily be confused with basalt, and a microscope would be necessary to distinguish the two.
Why do the lighter feldspar crystals often appear fuzzy or flat, stick-like in shape, in diabase rock? This is because they crystallized first, forcing the other, darker minerals to squeeze in around them, which distorted the feldspar. This is the opposite of what generally happens in rock formation; the dark minerals tend to crystallize first.
Colors can vary with diabase from gray to black, greenish black, and brown.
Pegmatite is an extreme plutonic igneous coarse-grained granite that forms during the final stage of magma’s crystallization. They are extreme because they contain exceptionally large crystals made of feldspar, quartz, and mica, as with granite. Many of the crystals range from several inches to a foot or more in diameter. It is the parent rock of many gemstones including topaz and tourmaline and rare and valuable minerals such as beryl.
Note: Even though the above pegmatite beach stone sample has large, coarse grains, Lake Michigan has ground it down, molding it into a round ball, demonstrating the powerful tumbling action of the wind, waves, and sands of the inland ocean.
Pegmatite is seldom seen in large masses but is usually found in veins cutting through other kinds of rock, such as granite and diorite. Pegmatite should not be confused with porphyritic granite, as the two can be distinguished by the relative size of the mineral grains. In pegmatite, the crystals are uniformly large, unlike porphyritic granite which usually one mineral is in large crystals within the finer ground mass.
Granite, Gabbro, Syenite, and Other Pegmatites
To indicate the mineral composition (or make things even more complicated), pegmatite can be “granite pegmatite,” “gabbro pegmatite,” “syenite pegmatite,” and any other plutonic rock name combined with “pegmatite” are possible. My first sample reminds me of the salt-and-pepper grains contained in diorite, so it could possibly be "diorite pegmatite."
Note: I featured diorite, gabbro, and granite in my photo essay about other types of beach stones, but will briefly explain them here because they have pegmatite forms.
- Diorite is primarily composed of feldspar and various dark-colored minerals, which explains its black and creamy white coloring with a salt-and-pepper pattern.
- Granite is composed of four materials: feldspar, mica, quartz, and hornblende minerals. These minerals themselves come in a variety of forms, giving granite a much larger variety than diorite or gabbro.
- Gabbro is sometimes called “black granite” for its similar coarse-grain appearance, but a large proportion of the iron-bearing minerals make gabbro heavier and usually darker-colored.
Conglomerate "Pudding Stones"
Conglomerates are sedimentary rocks with inclusions of rock pieces of various sizes and shapes cemented with sand and pebbles by dissolved minerals. Heat and pressure over long periods of geological time mold the mixture and hold it together. The pebbles and small rocks in a conglomerate are typically rounded, a feature that differentiates them from "breccias" where the larger stones in the mix are angular. Again, these "pudding stones" are not the most common rocks I find on the beach but are, nevertheless, a regular find. The second sample was found only near the beach which lacks some of the sanded-down smoother rock typical of beach stones.
These conglomerate samples are actually man-made, cemented together with tar for road construction. However, they find their way into our beaches with the telltale polishing from the wind, waves, and sand action!
Banded Metamorphic Stones
I've always been attracted to these banded beauties and have fun arranging them in my rock garden. The samples directly above and below are metamorphic slate transformed from sedimentary shale, but I haven't yet identified the others further below. Feel free to make a guess if you think you may know.
To briefly review metamorphic rocks: Metamorphism involves the alteration of existing sedimentary or igneous rocks by either excessive heat and pressure or through the chemical action of permeating fluids. This alteration can cause chemical changes or structural modifications to the minerals making up the rock.
Metamorphic Rock Traits
- Because their mineral grains grew together tightly during metamorphism, they're generally strong rocks.
- They're made of different minerals than other kinds of rocks and have a wide range of colors and lusters.
- They often show signs of stretching or squeezing, giving them a striped appearance.
Of the metamorphic rocks I have discussed, slate, gneiss, and schist have a layered or banded appearance, but metamorphic quartzite does not have a layered or banded appearance.
I leave you with beautiful photos of Lake Michigan beach stones shimmering in the shallows or washed over by the last wave!
© 2018 Kathi Mirto
Teresa on August 08, 2020:
Thanks so much for sharing your knowledge and experiences. Loved the article and photos too.
Debra Davidek Bobo on May 19, 2020:
Thanks. After a rain I find all these types of stones in my driveway and various places on our property. We are partial to all rocks. We live in the Boyne City, MI area; the rolling hills outside of town.
cool guys on February 26, 2020:
Love it so much helped alot.Thanks
Monstro on February 24, 2020:
Hello there Kathy. I love this article but wanted to let you know about one of you images. The image of Puddingstones is actually worn chunks of concrete. I found this post because a friend of mine was asking me for a mineral identification and used your photo to show me as an example of pudding stones.
Your article as a whole is very informative and well done. I just wanted to share this info with you.
David on September 22, 2019:
Thank you so much for your interesting, informative and helpful rock articles!
Cindy Fahnestock-Schafer from Hedgesville, WV on March 24, 2019:
Kathi, I really love this hub and thought that I would get tired of looking at rocks lol. but it didn't happen. I hope you will check out this. Sometimes, a rock is not just a rock and you are the type of person to find one of these lol. Thanks again. cindy https://owlcation.com/humanities/Bear-Spirit-Mount...
Del Banks from Southern Blue Ridge Mountains on December 06, 2018:
This is fascinating stuff, Kathi. I had spent considerable time in my youth following my mother (a real rockhound) around the beaches and deserts of the southwestern US, and may of the rocks that you pictured look similar. I never really gave much thought to the Great Lakes as a source for such a beautiful variety of stones. Great hub, Kathi. Very informative, and it's great to be in touch again! God Bless!
Kathi Mirto (author) from Fennville on May 21, 2018:
Thank you Linda for the comment and I agree, I like the last photo too!
Linda Crampton from British Columbia, Canada on May 21, 2018:
This is a very informative article. You've provided a great education for someone interested in rocks! I love your photos, especially the last one.