Concretions are one of those rock curiosities that rock hounds love to find. They are fascinating and tease the imagination. Since the early 18th century these fascinating rock formations have been considered one of the world’s geological curiosities and wonders. I’m a rock hound and I collect rocks and minerals and have a few of these marvelous rocks in my collection.
Concretions are a form of sedimentary rock and are a lot of fun to look for and collect. Wherever there is sedimentary rock you are likely to find a concretion. Concretions are not rare but they are distinctively unique.
One day
when you are walking on a hillside, in a meadow, along a river, stream or lake you may find something that is rock with an unusual shape to it that looks like a ball with bumps and rounded points on it or it may look like a petrified cucumber or squash, even a giant size pretzel that has been turned to stone, some snack left behind by some ancient civilization.Maybe you will find what appears to be a petrified turtle, bird or frog or maybe a mole, a giant insect of some sort or a twig or small branch from a tree that has turned to stone. It is probably none of these things and is not a fossil of any kind though these rocks are often considered to be pseudo-fossils; rocks with a fossiliferous nature or appearance. Concretions are not petrified or fossilized plants or animals of any kind.
In ancient times these concretions were thought to be extra-terrestrial and had legends of a supernatural, spiritual nature surrounding them; gifts from the gods. Such finds were both feared and revered. Sedimentation and the formation of sedimentary rock were not clearly understood in ancient times. Today we know they are fascinating creations of nature but we no longer fear them or even revere them except in a very loose sense of the word. We do appreciate them and may even be a little awed by the treasure we have found. Each one is unique in its very own way and no two will ever be exactly alike.
Concretions are made by water seeping through beds of rock like limestone or clay and dissolving the minerals such as iron, and calcite while picking up bits of sand and other minerals and redepositing the whole composite mixture around roots, twigs, leaves and other pieces of rock stuck in the muck that in millions of years will become sedimentary rock and cementing these deposits in place. As these deposits harden they take on various shapes that may fool us into thinking they are something they are not.
These sedimentary deposits usually become much harder than their host rock and are not as subject to erosion as their host is. As the host rock weathers these concretions are left behind for us to find and admire. Nature creates some marvelous masterpieces.
I have also found concretions that didn’t take a million years or even a thousand or a hundred to form. A few days of receding water, evaporation and hot sun did the trick. These are usually formed from clay deposits where rain causes the clay to splatter onto bits of other rock or twigs or other matter in the area or flood waters may leave such deposits behind and as the waters recede and the sun comes out these deposits are baked and dried into strange shapes. Concretions can be very large or as small as or smaller than your thumb nail.
One day I found a whole handful of them on the pitcher’s mound at a local ball field. A few of them cracked and crumbled easily while others were very hard. The hardness depends on the minerals left behind and the length of time they were given to form and harden. They are still concretions because of the way they were formed and are in my collection even though they are not thousands or millions of years old like some of my other pieces are. These are what I call “concretions in their infancy.” Left alone they would have either been dissolved by water seepage or would have grown to become that very hard, oddity called a concretion and thousands of years from now some other person may have found them right where I did as the host sediment eroded away to re-expose them. I found them first.
It is always fun and interesting to find a concretion but they really aren’t worth much monetarily except to the collector, while collecting good to high quality gemstones or fossils may well be.
Geodes are another of the very fascinating rock formations. Geodes look somewhat like a concretion or some concretions do but they form differently and are much harder than a concretion. They are also rarer; and then again, maybe not, maybe we just overlook them because we don’t really know what we are looking for. While concretions are fairly easy to find if you are in an area where there is a lot of sedimentary rock, geodes are not so common in most areas and form mostly in areas where there has been volcanic activity thousands to millions of years ago. It takes a long time for nature to create a geode.
These marvelous rocks are often passed by without a second look, mostly because they are so plain looking that they are often tossed aside as not much worth keeping. They are usually dull and not much to admire on the outside, just rock, so they get an “oh well, don’t think so,” and left right where you found them. It seems just another weathered and water tumbled rock but oh the beauty and the treasure you might have found inside if you had only looked. Inside you will find beautiful agate and cavities filled with sparkling crystals of quartz or calcite minerals.
Geodes are usually round like the Earth, (that is where their name comes from) or they may be more oblong, stretched out like a cucumber or oval like an egg and may be a bit irregular in shape depending on the type of hollow in the earth that they formed in. Geodes are usually a bit bumpy on the outside and feel much like running your hand over a head of cauliflower at your local farmer’s market but some may be relatively smooth with a more rough surface than bumpy if it has been brook tumbled or weathered for a greater time; and geodes range in size from about a couple of inches to gigantic, several feet in diameter. I’ve seen some of these really huge geodes in museums but have never found one to my knowledge. Most of mine are of the smaller variety from a couple inches to the size of an adult man’s hand; something I can carry and handle easily.
Geodes are most commonly formed in cavities and bubbles in volcanic rock but also form in other hollows in the earth, even in animal burrows that have been abandoned and where the environment is no longer disturbed. Geodes are created when
water seeps through limestone and gathers up bits of quartz and deposits the flow in these hollows where an outer shell is formed making a sort of bubble around the mineral laced solution that in time solidifies to form crystals in the cavity or fills it with agate or chalcedony. Over time, a lot of time, water dissolved minerals seep into these hollows or cavities in the earth and then dehydrate and harden into an outer shell; imagine it being the fossilized shell of an egg. The shell may not look like much but there is a lot of good stuff inside.The dissolved minerals continue to form inside this shell that they are now trapped in and form a beautiful crystal inner cavity; crystals of quartz, drusy quartz, amethyst, celecite, calcite, dolomite and the beautiful striated and wavy patterns of agate as well as the muted shades of chalcedony among others but these are the most common. The crystals all grow toward the center and eventually fill up the cavity.
Geodes are found throughout the world, commonly in places where there was once volcanic activity and a heavy quartz base to the land; but mostly in the western hemisphere and in some parts of Australia, Japan, China, India and Madagascar. In the United States you will find geodes in Arizona, California, Idaho, Illinois, and Iowa, in the Lake Superior region on both sides of the border, Kentucky, Michigan, Minnesota, Mississippi, Missouri, Nevada, New Mexico, Oregon, Wisconsin and Washington as well as in some parts of New York and New England and neighboring Canada.
Wherever there were once active volcanoes, there is a primarily quartz foundation and water seepage through limestone you may very well find geodes. Those old gray or dirty brown bumpy rocks shaped like an egg or a golf ball might just surprise you. Maybe it is not just a plain old rock. There might be real treasure inside but you will never know if you don’t open it.
Geodes may also be found where there is ancient coral limestone, like in Florida where mineral heavy water has dissolved the coral to form a cavity and quartz chalcedony has filled the hole much in the same way amethyst crystals might be found in geodes from New Mexico or agate and chalcedony lined geodes from Illinois or a few places in New England and most other regions mentioned. If you find a coral geode you have found a marvelous treasure.
Conglomerates, sometimes called Puddingstones are another fun find for the rock hound and are cousins to the concretion. Their name is exactly what it implies. Like a pudding made of various fruits, nuts and marshmallows in a base or batter like plum pudding for instance, conglomerates are a mixture of bits of various rocks and minerals all mixed up together in a solidified matrix or batter if you will of a host rock like sandstone or limestone; thus the name common name, puddingstone for a conglomerate. Conglomerates are sedimentary rock and really fascinating to study because you never know what you are going to find there.
Conglomerates are made up of large pieces (larger than sand particles, at least two millimeters in diameter) of crumbled and rounded rocks and minerals that have been broken away from their mother rock and carried away by water such as in a mountain stream or along a given path by glacial melt as ice melts and slides along its glacial path carrying bits of rock and mineral along with it. All these pieces get all jumbled and mixed up together and bound into place by sand silica or cement made of iron oxides, calcium carbonates and aluminum silicates that fill the cavities between the pieces of this stodge-podge of rocks and minerals and hold them in place to form a new rock called a conglomerate or puddingstone, a conglomerate that has gone through partial metamorphism.
These puddingstones usually contain large amounts of quartz and feldspar and other minerals and can be quite small or as huge as a giant boulder and are usually found along rocky beaches, streams, brooks and rivers. The fascinating thing about a conglomerate is that once you have determined what is in the pudding you will have a clue as to what types of rocks and minerals you will find upstream and the mixture can be amazing.
Conglomerates are pretty much worthless commercially because of their makeup though placer mining is done in some regions for this type of rock, mostly in Australia and the United States where the rock is used for decorative purposes. However, you can’t depend on their strength and durability. They don’t have a clean or smooth fracture, it doesn’t break cleanly, so they can’t be used as building stone like granite, marble or sandstone, so these beautiful and fascinating rocks usually end up in a gravel pit somewhere or are just left beside the river until a rock hound comes along or they get moved someplace else by floodwaters and winter run off or glacial melt. Puddingstones are usually very colorful and attractive rocks however and are sometimes used as ornamental purposes or conversation pieces.
I’ve seen these beautiful conglomerates as polished boulders used as centerpieces in indoor gardens in shopping malls. There is one I have seen in a cemetery that has been used as a grave marker that has a metal plaque attached to it telling who lies here. It turns out that the person buried there was a geologist; what a perfect grave marker; somebody was really thinking.
I don’t just horde the rocks I find. I try to find uses for at least some of them, like making rock critters or mosaics, crafty sort of things. I once crushed some puddingstone up and mixed it with clear shellac to use as a coating on an unvarnished, pine wood picture frame. It made a very interesting and pretty frame to mount a mountain scene that I had painted. It was a fun experiment but a lot of work. I sold the painting for a fair price so I guess it was worth it.
Rock hounding is a fascinating hobby, a sort of pseudoscience that ranges from mineral collecting and studies to fantastic curiosity rocks like concretions, geodes and conglomerates that can boggle the mind. I’m a rock hound and some of my favorite and most fascinating finds have been concretions, geodes and conglomerates.