You can classify most rocks on the bases of the formation process. Igneous, sedimentary, and metamorphic are their three classes.
Each of these classes can contain many types of rocks. It is essential to learn how to identify rocks to become a proficient rock collector.
To identify your rock, first make a note of its physical properties such as grain size, luster, layering, color, and banding. Next, test the rock for its weight and hardness by executing simple tests. As a final step, compare the properties of your rock to those of known rock types and look for other identifying features.
You will get details of this process here.
Related Article: Where to Find Rocks?
Analyze the Physical Properties of Your Rock
You need to analyze color, luster, grain size, shape, banding, laying, hardness, and weight because these are the physical properties of the rock. It can be your first step towards rock identification.
Some people think they can identify the rock when they have measured all these properties. It is not true because some of these measurements do not apply to all.
You need to look for many physical properties to get an idea about the identity. Combining these measurements with the rock origin information for accurate identification is better.
You can get rock properties information charts online. You can print these charts and match the properties. It will take a few minutes to check the physical properties and a few more minutes to compare them. Keeping the chart with you all the time is an excellent idea.
We will take a look at the following characteristics of a rock.
- Grain Size and Shape
- Color
- Texture and Feel
- Luster
- Layering
- Foliation or Banding
- Unique Identifiers
- Weight and Density
- Hardness
Related Article: Can You Bring Rocks on a Plane?
Grain Size and Shape
Grain size and shape can tell you many things about a rock. Most people think that only sedimentary rocks have grain. It is not true because all have grains in different forms.
Some are small enough to hide from the naked eye. If you have found a clastic sedimentary rock, the grains are visible as you can identify them from these grains.
You may need a magnifying glass to see the grains of mudstones because they will look smooth with the naked eye.
There are two categories of igneous rocks. If these rocks cool down rapidly after formation, they will have a smooth surface with small grains.
These rocks are known as extrusive rocks. When the igneous rock cools slowly, it will form bigger grains that you can see with the naked eyes. You can call these rocks intrusive.
You can also identify the metamorphic rocks by checking the grain shape and size. These rocks have a base rock known as the protolith.
You need to check the protolith and the rock above it to identify the rock as the rock may have a different name if the protolith and the metamorph rock have the same crystal direction.
There is a peculiar name when the orientation and size of crystals in both parts are the same. You need to check the grain size and shape of the grains to identify the rock.
The grain orientation can also give you valuable data. You can use this data for identification.
Color
Most rock collectors start the identification process by checking the color, as some beginners make assumptions about the name based on the color.
It is not an accurate way to identify the rocks. It is better to consider many other factors from the list of physical properties to find the identity.
Rock color can be deceiving because they can get some rock color on the surface due to impurities. Iron can give a red color to the rock surface. It can trick the beginners into believing it is a red-colored rock.
It is better to list all the colors you see on the rock. You can select one or two colors from the list that make most rock parts.
It is crucial to consider other physical properties before deciding about the identification. The rock can be dirty due to many surface impurities.
You can break a little upper rock layer for a better rock color view because it will help with accurate identification.
Related Article: Why River Rocks are Smooth?
Texture and Feel
It is hard to identify the texture of the rock because you need sensitive fingers to do that. This test can give different results when two persons perform it.
One may feel that rock has grains, while others may think it is soft. As there are minor differences in the texture of different rocks, identification is hard using this parameter alone.
You can take the rock and feel it with your fingers. You can check for the grain’s texture and grain size. The rock will have a smooth texture if the grain size is small.
You can identify the rock with a smooth texture as most of the extrusive igneous rocks are smooth. Metamorphic rocks can also have smooth surfaces.
Most sedimentary rocks have a rough surface with larger grains. You can learn this skill as you test more of them for identification.
Related Article: Is It Illegal to Take Rocks from Nature?
Luster
It is easy to identify a rock if it is glass-like or sparkly because only a few exhibit these properties. Most of them come under the dull rock category.
You can check the amount of light reflectability by observing it closely. It is easier to identify crystals and minerals with the amount of luster.
You will not get much information from the rock luster as most rocks are dull.
If you are making a list of physical parameters, you can write dull, glass-like, or sparkly in the luster section because you will not get more information from the physical checking of the luster at this point.
If your rock has metallic minerals, it will shine more. They may have magnetite or pyrite containing minerals that produce shine.
Your job will become easy when they have these minerals. Dull rocks require more struggle because you need many other factors to identify them.
Related Article: Best Gifts for Rock Collectors
Layering
Layering means that the rock has different layers of one millimeter to a few inches that run all over the rock at the same position.
Layers do not have to be straight as you can have curved layers also. You can see the layer pattern in the metamorphic rocks because they will show banding.
Layering is frequent in sedimentary rocks because different rock layers form under specific environmental conditions.
You can have the same crystal and grains with slight color variation in the layering. It is not the same as banding, where you will see different minerals in different rock layers.
It is hard to see the layering in the igneous rocks because the entire rock form from the molten lava. It forms with uniform crystals.
Related Article: What Rocks and Minerals are Worth?
Foliation or Banding
It is an indicator of metamorphic rocks. Banding means that the rock has small layers of different minerals. You need to know two important points before identifying based on these parameters.
- Foliation is the exclusive property of metamorphic rocks.
- You need to differentiate layering from foliation for accurate identification.
You can identify the foliation by checking the grains in the rocks. You will find compaction and elongation of the crystal grains. If there is no banding, you may have found an igneous or sedimentary rock.
The layering is different than the banding because you will not see layers of grains. Same grains can change color in the layering due to weather changes.
Related Article: Beautiful Minerals and Cool Rocks to Collect
Unique Identifiers
You can check the unique identifiers if you have narrowed down the type of the rock and want to confirm the rock’s identity. There are many unique identifiers to look for in rock.
Most of these identifiers are hard to find because you need an expert’s eye to find them. You can also use a magnifying glass to check some parameters.
If there is something special about the grain size, you can check it with a magnifying glass. You need to find something unique and abnormal about the rock. You can start by checking the bubbles in the rocks.
You can also look for fossils and fractures. It is also possible to check the unique identifier by using chemicals. You can use weak acids or bases to see how the rock reacts.
Some rocks produce specific colors with acids and bases. You can find the exact identity of the rock with these methods.
Weight and Density
There are two ways to perform this test on the rocks. You can do it with your hands or get equipment to get the exact value.
Most experts recommend that you do the test with your hand, as getting the specific gravity equipment will not make a difference. Many rocks can have different specific gravity and weight.
We can take the example of granite. The specific gravity of granite ranges from 2.7 to 2.8. The difference is due to the difference in the composition of the granite.
Granites consist of quartz, mica, and feldspar as they can have many other minerals in the structure. The difference in the quantity of these minerals is the reason for the weight difference.
You can pick the rock in your hand and try to measure the weight. You can compare the weight of this rock with others in your collection. You can write that it is heavy or light.
It is hard to make more parameters using this test as people will not judge the weight accurately. The advanced test involves the use of specific gravity equipment.
It is hard to carry that equipment with you to a remote location. Most rock hunters prefer the method of measuring with the hand.
Hardness
Hardness is an essential parameter in identifying the rocks, as some rocks are harder than others. You need to perform a test to check the hardness level.
You need a glass piece and a metal piece to test. You will also use your nails to test the rock hardness. You need to scratch your nail with the rock.
If the rock makes a scratch on your nail, you need to go to the next step. If it does not make a scratch on your nail, you can write the test results. You need to use the rock to scratch the glass.
If it produces scratch, you can move to the next stage. You need to do the same with the metal sheet. You can write your results when the rock makes a scratch on all surfaces, or it does not produce a scratch on one of the surfaces.
It is better to take them to the lab to perform hardness tests with better equipment. This method is to get an estimate of the hardness level. You can write the results from this method according to the following points.
- Very Soft: If the rock does not make a scratch on the fingernail, it is a very soft rock.
- Soft: Soft rocks produce scratches on the fingernails but not the glass or metals.
- Medium: Rocks with medium hardness can make a scratch on the glass and fingernails but not the metals.
- Hard: You can identify a hard rock as it makes scratches on fingernails, glass material, and metal surfaces.
How to Compare the Rock Properties to Known Rock Types?
You need to complete the first step of checking the physical properties. It is better to make notes of all the physical properties to compare them with the other rocks.
If you know some details about the rocks, you will find the class of the rock in the first step. We will give more detail for the comparison in this step to help rock hunters at every level.
It is better to download online resources to identify different rocks. You can compare your findings with the online resources to find the exact identity of the rock.
There are some cases where you cannot find the perfect match in the online resources; you can go back to the first step in this case. If your findings are the same, you need to think of other ways to identify the rocks.
You can use the inline forums to send you data with the rock picture. It is possible that your rock is a rare one and does not have much information on the online resources. You can also get help from the experts.
Igneous
These rocks form when lava cools on the ground or in underground places. These rocks can have different compositions because the molten lava makes these rocks.
If you find a fossil in a rock, there are high chance that it is not an igneous rock because they do not have fossils. It is hard for a fossil structure to survive in the lava. These rocks are resistant to acids.
You can test them by putting a weak acid on them. Most other types of rocks will react with weak acids to change color. If you put the rock in the water and it floats, it is an igneous rock.
There are no other rocks that can float on water. You need to identify two types of igneous rocks to go to the individual identification.
The rocks with smooth and shiny surfaces indicate that lava rapidly cooled to make these rocks. The other type of igneous rock does not have shiny surfaces because the lava cooled slowly.
You can feel the texture by rubbing your finger on the igneous rock structure. It will give you many answers.
- Extrusive: When the lava comes to the surface and cools rapidly, you will get these rocks.
- Intrusive: These rocks form slowly because the lava does not come to the surface for rapid cooling.
Metamorphic
Metamorphic rocks form when you change the sedimentary and igneous rocks with metamorphic agents. Many metamorphic agents can change the existing ones to form a new type of rock.
Heat and pressure are the most frequent metamorphic agents. These agents can change the mineral structure and composition to make something new. You can identify these rocks with two factors.
The protolith and the crystal outgrowth. It is easier to identify a specific type of protolith if you are an expert in rock identification of other classes. You can also identify these rocks with the help of foliation.
As these rocks have layers of different minerals, you can identify them quickly. It is crucial to find the subclass of these rocks for individual rock identification.
There are three sub-classes of these rocks. These classes are based on rock foliations.
- Granofels: These rocks do not foliation making it hard to identify them.
- Schist: You can identify these rocks quickly due to the foliation as they have stone foliations.
- Gneiss: These are the most abundant rocks with foliation greater than 5 millimeters. It also has bigger grains for easy identification.
Sedimentary
You can identify these rocks because of their grainy texture. Sedimentary rocks have grains attached firmly. The presence of iron determines the color of the sedimentary rocks.
If it has iron with a lower oxygen concentration, it will get a gray or greenish color. You will also get layers of sedimentary rocks with red color. This color is due to the iron oxide that formed with more oxygen.
You can also get a gray color in these rocks due to the dead animals and plants. You need to distinguish three types of sedimentary rocks to come near the individual rock identification.
- Clastic: These rocks form when nature attaches different types of rocks with the particular material that becomes part of the rock.
- Biochemical: Out of all other rocks, fossils are more frequent in the sedimentary rocks. They are specific in the biochemical class of these rocks.
- Chemical: These are made of unique materials; some minerals lose the water dissolution and attach to make a rock.
How to Utilize Available Rock Identification Resources?
Most people will find the rock identity using the first and second steps. If you are still unsure, you can utilize different resources for confirmation.
Most people like online resources because you get help from them at any place you want. Even if you have all the resources in the world, you cannot identify the rock without physical properties measurements.
Once your notes are ready, you can check different resources for confirmation. You can check some of the resources we found online.
- Rock and minerals book by Smithsonian Handbooks: This book is excellent for finding the identity of the rocks because it has colorful pictures. You may have to spend some time to find the specimen with the perfect match as the book has details of hundreds of rocks.
- Stone ID app: It is an app on Google Play to upload a picture of the rock to find its identity. They use artificial intelligence to compare images of different specimens. If they find a match in their system, they will let you know. It is hard to identify the rocks based on the images. The app may give unreliable results sometimes.
- Reddit: What is the rock page: This page is popular among rock hunters because they can upload the rock image to get opinions from thousands of experts. It can also be confusing as many people give different ideas after looking at the pictures. You will become confused because of the many options, as you cannot decide which one to believe.
Should I Get an Outside Opinion?
If you did not get the answer about the rock identity, you need to search more. You can take the rock to some experts in your area to find out more about it.
Getting help is suitable, as most experienced rock hunters need help sometimes. You should go to the local rock stores first. They may give you information about this rock.
You can show it to experienced rock hunters with years of experience. If nothing works, you can take the rock to the geology experts who do land surveys in your area.
Consider Alternative Explanations
Some of us will never find the answer about the rock identity because we are looking at the wrong place.
If you have found a gemstone, you need to go to the gem experts instead of the people with rock hunting experience. You cannot identify a rare crystal with rock identification methods.
It is crucial to use different tools to identify them. The reason you have not found the identity can be the wrong class of tests.