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Why chert is the best rock to collect
A show of hands...
Do you have a rock collection?
Rocks are free
Although there are many places where you cannot collect rocks (National Parks, private land, etc.) and some places where you must pay to collect rocks, there are countless acres of public lands where rocks and minerals are freely available for the hunting and taking. This makes rock collecting an ideal hobby for the frugal naturalist who loves to spend time outdoors. Rocks come in all colors, shapes, and sizes, and each geologist has their favorite type. Chert has always been my favorite rock...even when I was too young to know what it was. Here's the dirt on chert:
What is chert?
Chert is a form of micro-crystalline quartz that commonly forms by the precipitation of silica-rich or "siliceous" fluids. In other words, chert is made of tiny quartz crystals. Because chert is made of quartz, one of the hardest and most common minerals on the earth, it is extremely resilient and prevalent. While rocks made of less robust minerals such as calcite (limestone) tend to weather rapidly in the presence of slightly acidic water (caves), quartz is durable and erodes very slowly. For this reason, almost every creek that is loaded with sediment will have at least some quartz, often in the form of chert. Sometimes this chert has travelled thousands of miles from its source of deposition. What better rock to search for than one that is ubiquitous and persistent?
Chert is variable
Chert can form in many different ways including lava flows, the precipitation of siliceous fluids, or by cellular replacement of decaying organic matter. Sometimes chert forms in layers known as "beds" and other times it forms in spherical masses known as "nodules". A common way to find chert is in a rock known as a "geode", which is essentially a nodule that never fully filled and has void space in the center. Although geodes typically have coarser quartz crystals, there is usually chert and also other minerals such as calcite and dolomite within.
Chert is beautiful
Pure quartz is clear or white in color, but it is very susceptible to color alterations from minor elemental inclusions. Just a small percentage of elements such as iron or magnesium in the siliceous fluid can drastically alter the color of chert. These colored forms of chert often have unique names based on their color. For example, flint is a well known form of chert that is generally grey, black, or brown in color. Jasper, on the other hand, is chert that is brilliantly red, yellow, brown, or green. The gemstones agate, opal, onyx, and chalcedony are also types of chert. Chert can come in practically any color imaginable, and usually these colors are blended into beautiful layers caused by flow patterns during the deposition of a highly siliceous fluid. Each piece of chert is unique - the reliable excitement of picking up a piece of chert and discovering what patterns and colors it holds...
Chert is hard
Because chert is made of quartz (SiO2), a mineral that has a very tight crystal lattice and is therefore very hard (7/10), chert has the ability to etch glass. Another artifact of the hardness of chert is that two pieces make a very distinctive clicking noise when struck together. Once the sound is heard and recognized, it becomes a tell-tale way to identify chert and quartz from other minerals. The sound of two pieces of chert rubbed together is somewhat soothing and can be used to induce a mild form of meditation. While chert collecting, I often fill my pockets with the stone and create my own symphony of chattering chert with every step. For reference, a bag full of glass marbles makes a sound similar to a pocket full of chert. There are few sounds as pleasing as a pocket full of chert!
Make your own worrystone
Chert can be smooth
The micro-crystalline or “cryptocrystalline” nature of chert means that there are not preferential directions of fracture (known as “cleavage”) that the rock will break along. Like pure quartz crystals, chert is said to have “conchoidal fracturing” which means that it breaks along irregular, often smooth surfaces. As a piece of chert tumbles down a stream and is slowly broken down, the rock weathers into many unique and interesting shapes. The smooth surfaces are generally pleasant to rub as a “worry stone”. If a piece of chert is jagged, it can easily be weathered and smoothed down using a standard rock tumbler.
Chert can be sharp
Primitive man discovered that when exposed to directionally controlled strikes, it is possible to form chert to have very sharp edges. This was exploited by early man to develop some of the first tools ever constructed (spearheads and arrowheads). These enabled humans to safely hunt large animals from a distance using durable stone points that could be reused. To this day, flint-knapping is a skill that is taught by survivalists and historians alike. An additional benefit of chert is that when struck with metal or another piece of chert, it generates small sparks that can be used to start a fire.
Chert is important
Because chert is comprised of tiny crystals of quartz, it does an exquisite job of fossilizing organisms. In a well preserved chert fossil, it is not uncommon to be able to examine the organism on a cellular level. Much of what we understand about the evolution of life on Earth is due to the preservation of organisms in the fossil record. While fossils can be made with varying degrees of preservation, the most detailed and well-preserved fossils are generally formed when a siliceous fluid replaces the cells of a buried and deceased organism, turning it into chert. A form of chert found worldwide is commonly known as fossilized wood. The cellular replacement of organic tree pulp with quartz preserves the growth rings, cells, and bark of the tree and produces a dense and unmistakable rock.
Chert is the best!
The next time you find yourself wandering down a stream, driveway, or trail, take note of the rocks around you. If you look closely, you will almost certainly find chert strewn throughout. If you’re lucky, maybe you will come across a piece of chert with beautiful colorings or fossilized organisms. At the very least, you will find some interesting rocks that can be used as meditation aides, glass etchers, worrystones, firestarters, arrowheads, gemstones in jewelry, or just as splendid observational pieces in a rock garden. Good luck chert hunting!