Any good rockhound is bound to come across a rock that he or she has trouble identifying, especially if the location of where the rock was found is unknown.
The following tips and tables contain characteristics that will help you identify the most common rocks on earth. Once you've determined what type of rock you've got, look closely at its color and composition.
This will help you identify it. Start in the left column of the appropriate table and work your way across. Follow the links to pictures and more information. Still having trouble identifying your rock? It's more effective to get your question answered by an expert.
Share Flipboard Email.Unit 3 Quick Igneous Rock Identification
Table of Contents Expand. Rock Identification Tips. Rock Identification Chart. Igneous Rock Identification. Sedimentary Rock Identification. Metamorphic Rock Identification. Need More Help? Andrew Alden. Geology Expert. Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U.
Geological Survey. Updated February 24, First, decide whether your rock is igneous, sedimentary or metamorphic. They are usually brown to gray in color and may have fossils and water or wind marks.
They come in various colors and often contain glittery mica. Next, check the rock's grain size and hardness. Fine grains are smaller and usually cannot be identified without using a magnifier. In simple terms, hard rock scratches glass and steel, usually signifying the minerals quartz or feldspar, which has a Mohs hardness of 6 or higher.
Soft rock does not scratch steel but will scratch fingernails Mohs scale of 3 to 5. F oliation Grain Size Usual Color Other Rock Type foliated fine light very soft; greasy feel Soapstone foliated fine dark soft; strong cleavage Slate nonfoliated fine dark soft; massive structure Argillite foliated fine dark shiny; crinkly foliation Phyllite foliated coarse mixed dark and light crushed and stretched fabric; deformed large crystals Mylonite foliated coarse mixed dark and light wrinkled foliation; often has large crystals Schist foliated coarse mixed banded Gneiss foliated coarse mixed distorted "melted" layers Migmatite foliated coarse dark mostly hornblende Amphibolite nonfoliated fine greenish soft; shiny, mottled surface Serpentinite nonfoliated fine or coarse dark dull and opaque colors, found near intrusions Hornfels nonfoliated coarse red and green dense; garnet and pyroxene Eclogite nonfoliated coarse light soft; calcite or dolomite by the acid test Marble nonfoliated coarse light quartz no fizzing with acid Quartzite.If you're interested in rock collecting, you know that rocks you find in the real world rarely look like the polished specimens you see you rock shops or museums.
In this index, you'll find pictures of minerals like those you'll most likely encounter in your expeditions. This list starts with the handful of common minerals called the rock-forming minerals, followed by the most common accessory minerals—you'll find them scattered in many different rocks but seldom in large amounts.
Next, you'll see a set of rare or notable minerals, some of which are common in commercial rock shops. Finally, you can check out some special galleries designed to help you to identify your specimens. Rock-forming minerals are among the most common and least valuable minerals in the world. They form the basis of igneous, metamorphic, and sedimentary rocks, and are used to classify and name rocks. Some examples include:. Biotite—Black micacommon in igneous rocks. Calcite—The most common carbonate mineralmaking up limestone.
Feldspar—A group making up the most common mineral in the crust. Feldspar Gallery. Pyroxene —A group of dark minerals of igneous and metamorphic rocks. Quartz—Familiar as crystals and as noncrystalline chalcedony. Accessory minerals may be included in any rock you pick up, but unlike rock-forming minerals, they are not a basic part of the rock. If the rock also happens to contain the mineral titanite, the rock is still granite -- and the titanite is classified as an accessory mineral.
Apatite—The phosphate mineral making up teeth and bones. Barite—A heavy sulfate sometimes found in "roses. Goethite—The brown oxide mineral of soils and iron ore. Halite—Also known as rock salt, this evaporite mineral sits at your table.
Pyrite—"Fool's gold" and the most important sulfide mineral. This collection of minerals includes metals, ores, and gems. Some of these -- gold, diamond, and beryl for example -- are among the most valuable and coveted minerals in the world. If you find these in your rock hunting excursions, be sure to keep them safe.
Amethyst—The purple form of crystalline quartz. Benitoite—Very blue, very rare and weird ring silicate mineral. It isn't always easy to identify minerals, even if they're fairly common.
Fortunately, there are tools used by geologists to aid in identification. Special tests for luster and streak can help; so too can these galleries of relatively common minerals of different colors. Black Minerals. Blue and Purple Minerals. Brown Minerals. Green Minerals. Red and Pink Minerals.Agates are semiprecious gems that can be found all over the world, and they are common along the shores of Lake Superior and in northeast Minnesota and northwest Wisconsin. If you're on the hunt for some, the familiar orange and yellow banding is a dead giveaway, but noting the size and weight of a found rock can also help identify it as an agate.
Most people can recognize a polished agate, but identifying unpolished agate on a lake or in a freshly plowed field is a little tougher. Making that identification is something that any amateur geologist can learn to do.
Look for translucence in the stone. If the stone has been broken and you can see telltale traces of a quartz-like mineral along with the red, brown and orange color that comes with many types of agate, there is a good chance that you have an unpolished agate. Examine the stone for banding. If the rocky exterior of the stone is broken or worn away, check for banding, which occurs in most types of agate. The banding is a factor that is sought by many collectors and jewelry makers.
Heft the stone. Most agate pebbles feel heavier than they look due to their dense composition. You may wish to compare suspected agates with other stones that are lying close by. Check for a pit-marked surface on the rough stone. Agates sometimes form in igneous rock, and they might also have been surrounded by softer rock that has since eroded away. Both of these situations can leave the agate with some pitting. Feel for waxiness in the revealed stone.
When you see a crack in the stone or a place where the rocky exterior has worn away, slide your fingers across it. Waxiness is a sign that you might have an agate. Study the stone for conchoidal fractures, which are irregular fractures that occur in fine-grained materials such as glass and obsidian. These fractures are often curved, with a wavelike pattern, and they impart an irregular profile to the rock itself. Agates are prone to conchoidal fractures.
Use a flashlight to back-light the stone. This will help you find out whether there are any translucent edges that you might have missed. There are many stones out there that look like agates, but aren't. Flint, chert and jasper are all closely related to agate and can have similar banding, but remember that they are opaque, whereas agate is translucent. Measure the stone. The average agate is less than 3 inches in diameter. Copyright Leaf Group Ltd.View All Videos View Minerals.
Advertising Information. Gemstone Images A-Z. Agate is a banded form of finely grained, microcrystalline Quartz. The lovely color Agate is a banded form of finely-grained, microcrystalline Quartz. The lovely color patterns and banding make this translucent gemstone very unique.
Agates can have many distinctive styles and patterns, but each Agate is unique in its own habit, with no two Agates being the same. The images below show examples of both Agates in their native, unpolished state, polished Agates, and combinations of the two.
Alexandrite is the highly regarded color changing variety of Chrysoberyl. The color The color of Alexandrite changes under different lighting conditions. If viewed in daylight, its color is greenish blue to dark yellow-green.
If viewed in incandescent or candle light, its color is pink to red. Alexandrite is a very rare and highly valuable gemstone, and until recently was extremely difficult to obtain due to its rarity.
However, new sources in Brazil and Tanzania have made this gemstone available and more mainstream on the gemstone market. Almandine is the most common form of the gemstone Garnet. The term Garnet describes The term Garnet describes a group name for several closely related minerals that form important gemstones, and Almandine is an individual member mineral of the Garnet group.
Almandine is usually opaque and unfit for gemstones use; though the less common transparent to translucent forms make fine gemstones. In the gem trade, the term Almandine is rarely used on its own. It is either generically called "Garnet", or "Almandine Garnet". Amazonite is a translucent to opaque feldspar gemstone with a pretty green color. It is the greenish variety of the mineral Microcline. Amazonite is named after the Amazon River of South America.
There are no Amazonite deposits that exist in the Amazon region, so presumably this stone is named to its resemblance in color to the green of the tropical rainforest.This guide also includes information about how each rock was formed, the location from where they were collected and their uses in everyday life.
Year after year in-kind contributors from the mineral and aggregate industries provide valuable resources for our educational units. The sheer number and variety of rock and mineral samples required for the production of the units is immense.
More than 60 samples representing 25 different types of metallic and industrial minerals, aggregates and the three main rock groups — igneous, sedimentary and metamorphic — are required for each kit. The organization relies greatly on their industry partners, resident geologists from the Ontario Ministry of Northern Development and Mines and a crew of students to aid in the collection and sizing of samples. We would like to take this opportunity to thank the many in-kind contributors for their on-going support.
However, it flows much quicker because it is less viscous. The Hawaiian Islands are made of basaltic lava.
The Most Difficult Rocks to Identify
The ocean floor is also mostly basalt. Basalt fibres are used in the production of high quality textile fibres, floor tiles, basalt plastic reinforcement bars, basalt fibre roofing felt and glass wool fibre glass.
If the fragments embedded in the matrix are angular instead of rounded, the rock is called a breccia pronounced BRECH-i-a. Rock Type : sedimentary Composition : dolomite and fossils Metamorphoses to : marble Environment : Sea water, high in magnesium, flows through porous limestone and replaces some of the calcium with magnesium turning limestone into dolostone.
Fossils are plants or animals that have been preserved in rock as organic carbon, chitin, or some mineral that replaced the original tissue. When an animal or plant dies its body can end up being buried by mud or other sediments.
The hard parts skeleton, teeth, shell and sometimes tissue leaves, flowers, muscle, cartilage may be preserved when the sediments become rock.
Distinguishing Characteristics : Grey with fossils that are visible. Anything that looks like it was once alive may be a fossil. Fossils are often the same colour as the rocks in which they are found. It is found in asphalt mixes for roads and streets, high strength concrete mixes used for high-rise residential buildings, bridge overpasses, sidewalks and airport runways.
Crushed dolostone is used to create drainage layers under high volume roads and is found in uncontaminated construction fill. Often chromium, nickel and platinum occur in association with Gabbro.Out of a million rocks on a beach, a child will select the most unusual. Keep this in mind if you visit an elementary school! If you are highly skilled at rock identification, I am willing to bet that there is a location near your home where your hand-specimen identification skills can be put to a rigorous test.
The location isn't an outcrop. It's your local elementary school. There you will encounter a diversity of interesting rocks - many of which you will be unable to identify. It does not matter how many petrology courses you have taken or how many outcrops you have studied. You will probably be caught off-guard by what students bring to school. I've done many "visiting geologist" lessons at elementary schools, and my first one remains the strongest in my mind.
I was there to teach a lesson on drawing volcanoesand the teacher told me that her students brought a few rocks for me to identify. After the volcano lesson, rocks started appearing out of pockets, lunch bags and desks. I expected them to be an assortment of the local rocks and fossils. Instead, the rocks that they presented would have brought grins to the toughest Ph.
Instead of seeing one or two that stumped me, there were one or two that I could identify with confidence. The rest were some of the most unusual rocks that I have ever seen!
Few things bring on a professional sweat more than being in front of an audience and in over your head. To find yourself in that situation with third-grade students is a humbling experience. Rounded, shiny, colorful rocks attract children's attention. How many of these can you identify on sight? Guess what happened during my second visit to a K classroom? More difficult rocks. Upon questioning the students, I learned that some of them were collected locally, some were collected on vacations and some were given to them by family members who live far away.Lake Superior Agate Festival.
In a previous page, Part 1, I offered pictures and descriptions of common beach rocks found on Lake Superior beaches. In this page, Part 3I continue to show and describe more varieties of rocks and minerals that make attractive gemstones.
I also show here some recent agate finds and some agate-wanna-bes that some will mistakenly identify as an agate. Karen Brzys booklet on "Understanding and Finding Agates" was helpful in identifying true agates from agate first cousins or agate wanna-bes.
Clicking on the small images shown here will bring up a larger view in a separate window. Once there you can click on it and print it out. Agate Cousins - Agate Wanna-Bees.
I initially thought this was a water level agate but because the bands appear more opaque rather than translucent I think not. The bands are formed like they do in an agate but the crystals of the bands are so small they are opaque. Here's another example of a banded rock that isn't an agate. Quartz has filled a seam in a basalt rock. Here's three quartz occlusions where jasper has been fused with quartz with some agate-like banding around the margins.
Some would consider these true agates. Here is another example of opaque bands formed in a metamorphic rock. Gneiss is abundant in the Upper Peninsula beaches. Here we have translucent bands alternating with some that look more opaque. However from what I understand exists with a true agate, the bands are too broad. So I'd say this is another wanna-be. I believe these specimens are prehnite. The specimen on the far right may have some copper staining.
These rocks are Jasper, a microcrystaline form of quartz that has been stained, usually by iron ore.
The Gallery of Minerals
The rock on the right is Jasperlite with bands of iron ore. These banded rocks are slate, another metamorphic rock. Originally a layered sedimentary rock but was later subjected to heat and pressure to fuse the layers together. Slate are smooth and usually flat. Make good skipping stones. Part 2. More pictures and descriptions of other types of rocks found on Lake Superior beaches plus a few pix of Lake Superior Agates.
See more Lake Superior Agate Pictures plus some advice on using tumbling pellets to tumble and polish agates. Black River Harbor Agate Hunting - pictures and video. Karen Brzys. Includes over photographs and diagrams. Is this an Agate? This is required handbook for rock pickers who love to comb Lake Superior beaches looking for pretty rocks and hoping to find an agate.