Rock types! If you are not sure what kind of rocks are near you, the first thing to do is contact your state Geological Survey. You can order geologic maps of almost anywhere in the country.
Aside from that, let's plunge right into the three main categories. I'll include some of the major types of each, and some names you're likely to see bandied around recklessly
|IGNEOUS: Granite, Diorite, Gabbro, Basalt, Diabase, Rhyolite, "Pegmatites"|
|METAMORPHIC: Slate, Schist, Gneiss, Phyllite, Quartzite, Marble, Skarn, Hornfels|
|SEDIMENTARY: Sandstone, Shale, Limestone, Mudstone, Chert
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Granite: Don't know what it looks like? Go to your local graveyard and look at the polished tombstones that maintain their new look almost indefinitely. Some of them might be a hundred years old and still appear new. Notice the speckled composition. Yep... granite-- or a similar plutonic rock (see below). It looks a little rougher when it's not polished, of course.
Granite consists mainly of quartz, feldspar, and ferromagnesian ("dark") minerals: hornblende, augite, and biotite (though not necessarily). The overall color of granite is due mainly to the feldspar: pink, gray, greenish, white, and even bluish. Black-looking "granites" get their color from a high percentage of hornblende or other dark mineral; but by this point they are not really granites anymore (see "diorite" and "gabbro").
The granite specimen above has one end polished to show the grains. It is primarily a mixture of feldspar and quartz. The feldspar is pink, possibly microcline; notice there is more feldspar than quartz (just by looking at it, I'd say the quartz percentage falls within the 20-35% range, which describes true granite). The dark grains are "accessory minerals"-- as best I can tell, they are magnetite and zircon in this specimen.
In conversation, most intrusive, plutonic rocks are [incorrectly] lumped into the category of "granite" (porphyritic granite, gabbro, syenite, diorite, granodiorite, etc.) These rocks form a gradation, depending upon the amount of silica and dark minerals present. True granite consists of 20-35% quartz, more than 65% total silica, and generally more K-feldspar than Na-feldspar. Total percentage of muscovite, biotite, and amphibole (typically, hornblende) can be up to about 20%.
Granite is formed at some depth within the earth's crust, from magma intrusions. Longer cooling times translate to larger grain sizes. For collecting, larger grain size also means better specimens. You won't find many interesting specimens in the fine-grained "microgranite" boulders that sit on the lawns in north Jersey, for example
Granite deposits are exposed over geologic time by weathering and erosion of the less-durable rocks that cover them. Granite does not just flow out of a volcano onto the surface; if it did, it wouldn't be granite, it would be rhyolite.
Diorite: May be incorrectly called "granite". Diorite contains virtually no quartz, no muscovite, and no K-feldspar. It looks a bit lighter than gabbro, sometimes resembling gray granite. The difference is that there will be no visible quartz grains. Remember, K-feldspar can't be distinguished from Ca- or Na-feldspars with the unaided eye. Diorite consists of as much as 50% dark minerals (ambhibole & pyroxene).
Gabbro: May also be incorrectly called "granite" or "black granite", but contains absolutely no quartz, no muscovite or biotite, and no K-feldspar. Gabbro consists mainly of pyroxene (i.e., augite), olivine, and Ca-rich plagioclase feldspar (labradorite or bytownite). Tiny amounts of amphibole (i.e., hornblende) may be present. The next step in the plutonic rock gradation is peridotite, a rock with an even higher percentage of olivine and very little feldspar.
Diabase: this is actually a variety of gabbro (see above) which consists mainly of labradorite feldspar, augite, magnetite, and olivine. Its grain size is on the smaller side, with a very strong interlocking structure of crystals which are responsible for its toughness. In the building trade, diabase is known as "traprock" and is quarried for driveway stone and railroad ballast. Diabase and gabbro have the same general mineral composition as basalt, but they have a larger grain size.
Basalt: chemically, this is identical to diabase and gabbro, but is very fine-grained. Basalt is from a melt that cooled very rapidly- in other words, on the surface of the earth or in the ocean. When basalt is broken, it leaves very sharp edges. The finer the grains, the sharper these edges can be-- with the most extreme being obsidian (volcanic glass), which can form from either basaltic or rhyolitic lavas.
Basalt, like diabase, is often known as "traprock" in the building trades. I read somewhere that this kind of "traprock" is from a slightly altered basalt. It probably doesn't matter; either stone is extremely tough and has a very high resistance to crushing, which is good for stones that will support cars, trucks, and railroad tracks.
Rhyolite is an igneous rock of volcanic origin. We normally think of rhyolite as something that occurs only in the western and southwestern USA, but rhyolite does occur in Maine and other eastern states-- it's just not as common. While basalt usually originates from a very thin, easily-flowing lava, rhyolite comes from a very viscous and slow-moving lava. The difference is in the chemical / mineralogical composition. Both rhyolite and basalt are "extrusive igneous rocks" because they came from melts that cooled on the earth's surface rather than deep underground.
Rhyolite has a high silica content and is actually the same mineralogical composition as granite. Rhyolite is to granite what basalt is to gabbro. Rhyolite is "aphanitic", meaning the individual grains are so small you can't see them without a microscope.
Rhyolitic lava flows do not seem to be as abundant as basaltic lava flows. Because rhyolitic magma and lava are so thick, they tend to build up pressure until a gas explosion in the volcano throws them all over the place in particles ranging from ash all the way up to big blocky chunks. (Thinner rhyolitic flows can cool very rapidly and turn into obsidian, although basaltic flows can also yield obsidian. This is because "obsidian" refers to the glassy nature of the rock, not to its mineral composition.)
Some rhyolite deposits have interesting color and flow banding. These are sought after by lapidary enthusiasts as a gem rock. "Wonderstone" and "Hickoryite", both found in Mexico, are good examples of this. "Wonderstone" also occurs in Nevada, Utah, New Mexico, and elsewhere. I saw some from Nevada that was called "Liesegang stone"; it had really beautiful purple banding on a lighter background. Such lapidary-grade varieties of rhyolite can look quite a bit like jasper at first glance.
Marble: This important metamorphic rock is used in the building and agricultural trades, but it also contains many interesting minerals if impure. Marble results from metamorphosis and / or metasomatism of limestone, which is a sedimentary rock. Marble is essentially calcium carbonate (calcite), but it may be dolomitic (containing magnesium) or have other impurities and intrusive bodies which make it very confusing. A good example is the Franklin Marble, the subject of much study.
Skarn: This is a metamorphic rock that can form in several ways through alteration of country rock. The original rock can be igneous or sedimentary, but the end product would still be called a "skarn". The alteration process can occur where there's magmatic intrusion, faulting / shearing, hydrothermal activity, or some combination of these. Skarns are typically of "calc-silicate" composition and contain garnet and pyroxene. The term "skarn" is often used in conjunction with, and sometimes as a replacement for, hornfels .
"Skarn" is typically used to describe rock where there has been chemical exchange between an intrusive magma body and the surrounding country rock; this kind of alteration is called metasomatism. "Hornfels" seems to refer to rocks that have simply been "baked" by the heat of an intruding magma body, without the chemical exchange between the two; this type of alteration is called contact metamorphism.
Depending on the degree of metamorphism, skarns can range from less than an inch thick to many hundreds of feet.
It is not difficult to find rocks at Franklin, NJ which contain a characteristic assemblage of augite / diopside, rhodonite / bustamite, and andradite. We commonly call these "skarn boulders" when they are large. These rocks may also contain other minerals such as willemite, tephroite, fluorapatite, and calcite. The Franklin complex has a very intricate geologic history which hasn't been fully deciphered, but it seems pretty safe to refer to it on the whole as a "skarn deposit". Metasomatism definitely played a huge role at some point in Franklin's geologic past.
Slate: when shale or mudstone undergoes mild regional metamorphism (that is, mostly due to pressure from overlying rocks), the result is slate.
Slate, like shale, can be split along bedding planes-- but thin pieces of slate are not nearly as crumbly as shale. They're not indestructible, but with a little care they have a number of uses. These thin "slates" have been used for chalkboards, roofing tiles, walkway stones ("flagstones") and other purposes throughout history.
Phyllite: This is a metamorphic rock that's somewhere between slate and schist in its degree of change from the original rock. It is flat like slate, except the surfaces have quite a bit of mica in them. I have seen some patios made with phyllite instead of slate. If the pressure in the ground had been intense enough, the phyllite would have instead become schist.
Schist: This is a metamorphic rock that is more severely metamorphosed than slate or phyllite. It is more massive and more completely micaceous than phyllite. May contain feldspar and other minerals, but too much feldspar and it's not schist anymore, it's gneiss. Schist occurs in great abundance in Connecticut, USA. Schist may contain other minerals, including garnet, sometimes in large crystals.
Schist and gneiss deposits can be physically very close to each other. One chunk of rock might be schist, and ten feet away in the same outcrop you could find a chunk of gneiss. It all depends on the conditions in which the rocks formed, and what was in them before they underwent metamorphism.
Gneiss: May be confused with schist, but is closer to granite in composition: more feldspar, less mica. It often has a warped or wavy appearance, resulting from immense pressure and heat acting upon igneous rocks at great depth.
Mudstone: a crumbly sedimentary rock which is a step below shale in compactness. Mudstone results from the compaction of silt under pressures that weren't quite enough to make shale. It may contain fossilized plant fragments, etc. Unfortunately, mudstone can fall apart quite easily, meaning that fossiliferous mudstones sometimes have to be stabilized if they are to be kept as specimens.
Shale: at some point in geologic time, surface rocks eroded and weathered, releasing silt and clayey sediments over the course of millions of years. Next, these sediments built up in very thick layers. Finally, when the accumulated layers became heavy enough, the lower sediments compacted and became cemented together to the point where they formed shale. Geologic upheaval or further erosion exposed these shale beds on the surface, where we find them now.
Shale is easy to split along bedding planes, resulting in a bunch of flat pieces. These pieces aren't very strong and usually break into small fragments at the slightest impact.
Shale can have calcite and a couple of other low-temperature minerals which were introduced by mineralized water solutions flowing through cracks. Shale is not the best kind of rock for mineral prospecting, but it's sure great for fossils.
Sandstone - like shale, this is a clastic or detrital sedimentary rock-- originating from the weathering and breakdown of other rocks. Sandstone was formed from grains of sand instead of silt or clay, so its final appearance is that of a rock made of compacted sand. You can see these sand grains with a magnifying lens, sometimes without one. Very compact sandstone is sometimes used as a gem rock if it can be made to take a good polish. Some of this material can be called "Wonderstone", but it is an entirely different rock type from rhyolite. The banding in sandstone is usually from bedding of different sediments. This kind of sandstone is also called "picture sandstone" if the patterns are reminiscent of sand dunes or other landscape.
If sandstone becomes compacted and heated enough in the depths of the earth, it forms the metamorphic rock known as quartzite. The best way to distinguish between quartzite and hard sandstone is by looking at broken surfaces under magnification: sandstone breaks around the grains of sand, but quartzite is so tough that the fractures go right through the sand grains (if you can even tell them apart).
Limestone is composed of calcium carbonate (CaCO3), thought to originate from the carbonate-based shells and skeletons of tiny sea creatures. Limestone can range from chalk-white all the way through dark gray to almost black. This dark limestone looks like basalt or "traprock" at first glance, but it is much softer and is easily attacked by acids.
Limestone can be rich in fossils, usually marine in origin. Limestone beds can have caves and underground caverns in them, thanks to the slow solvent effects of groundwater. This can cause sinkholes if the underground caverns are too close to the surface. Some limestone cavities are full of crystals of calcite, celestite, or other low-temperature minerals. Even when there are no caverns nearby, limestone specimens often have small seams filled with calcite. Sometimes this calcite is fluorescent, and if you're really lucky you can find actual crystals of calcite if the seams are wide enough.
Travertine is a chemical sedimentary rock that has the same chemical composition as limestone. This rock is also known as "Mexican onyx", because large deposits of it were found in Mexico and were used as ornamental stone. (This is a good example of how colloquial names can be very confusing).
Chert is a sedimentary rock which can be either biochemical or inorganic-chemical in origin.
Chert originating from the biochemical route is thought to form either from direct precipitation of silica from seawater or from the fossilized silica-rich skeletons of tiny sea creatures such as sponges, diatoms, and radiolarians. This chert occurs as nodules inside limestone deposits and may be exposed on the surface by weathering.
Chert suspected of forming through the inorganic-chemical route is found near areas where there was volcanic activity. The theory is that hot, silica-rich fluids leach out of magma and into surrounding rocks, where the silica precipitates upon hitting the much cooler areas. Further changes may occur on this siliceous deposit due to pressure and re-heating. When we consider this volcanic origin, it sounds as though this chert is something more than a "sedimentary" rock.
Chert is usually white, gray, or tan, but some varieties have interesting and beautiful color combinations such as swirls of purple and red. "Mozarkite", the state stone of Missouri, is a perfect example of this. This type of chert is highly sought by lapidary enthusiasts as a gem rock. Chert was also favored by native North Americans for arrowheads and spear points.
Chemically, chert is mostly silica (SiO2) in cryptocrystalline form, meaning the crystals or grains are so small that they are "hidden"-- you cannot see them without a very powerful microscope. Flint is another name for chert that usually describes black or dark gray material, and jasper seems to denote a chert or chert-like rock that has a reddish coloration, thanks to iron impurities. There are plenty of "jaspers" that don't have any red in them, though. Some jaspers are white, yellow, green, tan, gray, pink, or even purple.
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