How to Identify Your Rocks: Full Guide With Helpful Tools

One of the most common frustrations we have as rock collectors is identifying the cool rocks we find. It’s fun to discover a cool-looking rock and add it to your collection, but it can often be difficult to know exactly what it is you’ve found. I have found that there aren’t a lot of great resources out there to help rockhounds identify their rocks, so I decided to put together this guide to aid you in figuring out what kind of rocks you’ve found.

To identify your rock, first take note of its physical properties like color, luster, banding, layering, and grain size. Next, test for hardness and weight by running simple tests. Finally, compare the properties of your rock to those of known rock types while looking for other identifying characteristics.

Identifying and classifying rocks can be a tricky business. I have literally written the book on practical rock identification (link to Amazon), but in this article, I’ll go through the complete process you should use, point you toward some useful tools, and give you some generalized pictures for comparison.

Analyze the Physical Properties of Your Rock

Rocks are classified based on their origin and their physical properties such as mineralogical composition, grain size, and texture. Observing and, in some cases, measuring these physical properties is the first and most important step toward identifying your rock.

Tip: If you’re interested in learning to identify minerals, check out my complete mineral identification guide.

Go through each of these identifying properties and write down your observations. Depending on what type of rock you have not all of these properties will be applicable, but most of them will be.

Once you have gone through this list of physical properties (it should just take a few minutes), consult the tables I have compiled below to find the best match for your rock.

Color

One of the first things you’ll notice about any rock is its color. The color of a rock is often overrated when it comes to identification, but it is certainly an important characteristic that should factor into the process.

Your rock may be one uniform color or it may have multiple colors. Make note of every color you see, but you’ll want to place the most emphasis on the most predominant one or two colors. The different colors are due to the different minerals that your rock is comprised of, and by getting a rough estimate of the percentage of each color in your rock you’ll be able to make some inferences about its mineralogical makeup later on in this process.

TIP: If you’re serious about your rockhounding hobby and you want to learn how to identify rocks as well as most trained geologists, I’d highly recommend that you check out my Practical Rock Identification System. This bundle of information includes my eBook on the subject, informational videos, and powerful online tools to help you identify rocks like a pro.

Sometimes the color of a rock can be deceiving. Many people who are new to rock identification tend to lean too heavily on the color of a rock when trying to determine what it is. Always remember that color is just one of many important physical properties and weigh it accordingly.

It’s also important to make sure you’re looking at a relatively fresh surface on your rock. Some rocks will appear very different on the outside than they do on the inside because the exterior has been weathered and altered by the elements. If possible, break off a small part of your rock to expose a fresh surface – especially if it looks heavily weathered on the outside.

Luster

Luster is a property that is more commonly used in the identification of minerals. It describes the way light reflects off of a surface and can be very useful for description and identification.

In the case of your rock, we won’t be using most of the terms and descriptions used for mineral identification. I just want you to take note of how your rock generally seems to reflect light. Is it glassy? Dull? Maybe a little sparkly?

Most rocks will fall under the category of ‘dull’, but there are a few rock types that are easily identifiable based on glassy or slightly sparkly appearances.

Grain Size & Shape

One of the most important observations you should make about your rock is about the presence, size, and shape of its grains.

When people think about rocks with grains they usually think about sedimentary rocks. Clastic sedimentary rocks will often have visible grains of varying sizes, but in some rocks like shales and mudstones the grains are too small to see with the naked eye. You may need a magnifying glass or gem loupe like this one from Amazon in order to see very fine grains. Most chemical and evaporate sedimentary rocks will not have visible grains.

Igneous rocks are subdivided into two categories that are expressed by grain size. Intrusive igneous rocks generally have large, visible crystals because they cooled underground and the crystals had time to organize and grow. Extrusive igneous rocks cooled more rapidly on the surface, and therefore they won’t have any visible grains.

For metamorphic rocks, the presence and size of grains is mostly determined by the protolith (the original, unaltered rock) and the degree of metamorphism. Sometimes when grains are present in a metamorphic rock they are the same crystals as were present in the protolith, but other times they are completely new crystals with different mineralogies, sizes, and orientations.

If visible grains are present, take note of the size distribution. Some rocks will be made up of grains that are all the same size (or very close), while others will have a variety of grain sizes. In sedimentary rocks, we refer to uniform grain sizes as ‘well-sorted’ while rocks with a variety of grain sizes are referred to as ‘poorly-sorted’.

It’s also important to observe the shape of the grains. Grains can be round, sharp or angular, or everything in between. This property is particularly important in the description of sedimentary rocks.

Foliation or Banding

The presence of foliation or banding is a very strong indicator that your rock is metamorphic. The rock will display what looks like banding, caused by the compaction and elongation of crystal grains.

This type of banding is only present in metamorphic rocks, so it can really help narrow down what type of rock you have. If there is no foliation or banding present then it is much more likely that you have a sedimentary or igneous rock, because the grains are still more or less unchanged from their original shape.

It is important to distinguish between foliation and layering. Foliation refers to the orientation of elongated crystal grains. All of the grains will be stretched, and they will be lined up in one direction. If you look closely you will see that there are no consistent layers that run through the rock. Instead, the rock is fairly uniform with the elongated grains interlocking with one another.

Layering

This leads us to the next important physical property to look for – layering. The layers you’re looking for are parallel beds that may be straight or curved. The layers may be very thin (about 1mm) or up to several inches thick.

Most layered rocks are sedimentary, but some are metamorphic. Layering is seen only rarely in igneous rocks. Sedimentary rocks are often layered because of the way they are formed, and metamorphic rocks can sometimes be layered if their protolith was. The presence and appearance of layering can be a big clue when trying to identify your rock.

Hardness

The hardness of a rock can be hard to quantify because most rocks are made up of several minerals which all have different hardnesses. Still, some basic tests can help narrow down what kind of rock you have.

In order to perform these simple tests, you’ll need a piece of glass and a steel nail (or other piece of steel). You’ll also be using your fingernails, but I’ll assume you already have those.

You can determine the general hardness of your rock by whether or not it can scratch or be scratched by your fingernail, glass, and steel. Try to scratch one of your fingernails with the rock. If it leaves a scratch, move on to the glass and do the same thing (keep the glass on a flat surface and take care not to break it). If your rock scratches the glass, move on to the steel and see if the nail will scratch the rock.

• Very Soft Rocks: Won’t scratch your fingernail, glass, or steel
• Soft Rocks: Will scratch your fingernail, but won’t scratch glass or steel
• Medium Rocks: Will scratch your fingernail and glass, but won’t scratch steel
• Hard Rocks: Will scratch your fingernail, glass, and steel

Weight and Density

For purposes of identifying your rock, all you usually need to do is take note of how heavy the rock feels compared to other rocks its size. When you pick it up, does it feel heavier than normal, lighter than normal, or about like you’d expect?

If you really want to get fancy you can perform specific gravity tests on your rock, but this is usually unnecessary. Because the mineralogy of rocks of a single rock type can vary, the density and weight will vary correspondingly. For example, the specific gravity of granite typically varies from 2.7 to 2.8 depending on the amount of quartz, feldspar, mica, and accessory minerals present.

Again, for our purposes, just hold the rock in your hand and see if it seems unusually light or heavy. This is usually enough to help with the identification process.

Texture and Feel

This test is a pretty easy one, but it’s also a little subjective. Run your fingers over the surface of the rock and see what you feel. Some rocks have textures that are unique enough that they can aid in identification.

Most rocks will simply feel coarse or rough, but if you feel something different be sure to make a note of it. Some rocks may feel glassy, slippery, greasy, or gritty. If you don’t feel anything notable that’s okay, too.

Unique Identifiers

There are all sorts of other unique identifying characteristics that can help you identify your rock. Most of them are pretty specific and aren’t seen in very many rocks, so I’ve grouped them together here.

We’re looking for anything abnormal or that seems distinct about your rock. Things like bubbles in your rock, the presence of fossils, a reaction with weak acids, or interesting fracture trends can all help you figure out what sort of rock you have. These types of features are especially useful if you’ve already narrowed down your possibilities and you’re looking for anything else that can refine your choices.

Compare the Rock’s Properties to Known Rock Types

Now that you have analyzed your rock and made note of as many physical characteristics as possible, it’s time to actually find out what it is! You may already have a very good idea of whether your rock is sedimentary, igneous, or metamorphic, so I have divided the tables below accordingly.

Use these tables to find the rock type that most closely aligns with the properties of your rock. If you find a rock that is very close but doesn’t quite match, it may be worthwhile to go back and re-evaluate the physical properties that don’t align. Also, keep in mind that this list is far from exhaustive.

These tables are pretty comprehensive, but there may be uncommon rock types not listed here. Follow the links for more complete information on each rock type.

Sedimentary

Sedimentary rocks are made up of mineral particles or rock fragments that have been cemented together. They are usually readily discernable from igneous and metamorphic rocks by the presence of grains, layers, and/or fossils. You can find my post dedicated to sedimentary rock identification here.

Rock Name	Composition	Grain Size	Hardness	Other Characteristics
Sandstone	Quartz, Feldspar	Medium	Hard	Many colors, mostly brown, tan, white, red, yellow
Shale	Clay minerals, Quartz	Very Fine	Hard	Thin layers, usually gray to black, fissile
Siltstone	Quartz, Feldspar	Fine	Hard	Many colors, not fissile, lacks clay minerals
Mudstone	Quartz, Feldspar	Very Fine	Hard	Not fissile, no layers, sometimes refers to limestone
Limestone	Calcite, Aragonite	Very fine	Soft	Often contains fossils, fizzes with acid, usually white or gray
Dolostone	Dolomite	Very fine to medium	Soft	Often confused with limestone. Only slight fizz on fresh surface
Breccia	Varied rock fragments	Fine to very large (varied)	Soft to hard (varied)	Similar to conglomerate, but with angular grains
Conglomerate	Varied rock fragments	Fine to very large (varied)	Soft to hard (varied)	Similar to breccia, but with rounded grains
Arkose	Feldspar, Quartz, rock fragments	Medium to coarse	Hard	Usually gray or red, similar to sandstone, but more feldspar
Greywacke	Quartz, Feldspar, rock fragments	Very fine to coarse (varied)	Hard	Dark gray color, angular grains, many grain sizes
Wackestone	Calcite, shell fragments	Very fine	Soft	Grains supported in mudstone
Coquina	Shell fragments	Medium to large	Medium to hard	Mostly shell fragments, can be poorly- or well-cemented
Evaporite	Halite, Gypsum, etc.	Coarse	Very soft	Rock formed from soluble minerals and evaporation
Chert	Quartz	Very fine	Hard	Many colors, very fine grains, conchoidal fractures. Includes Flint
Oolite	Calcite, Aragonite	Medium to large	Soft	Made of round ‘ooids’, can be white, gray, reddish

Igneous

Igneous rocks are formed from the cooling and solidification of molten magma or lava. Their composition is determined by the chemical composition of the lava or magma from which it forms, while the degree of crystal formation is determined by the amount of time it takes for the magma or lava to cool. In addition to this article, you can find my post dedicated to igneous rock identification here.

Rock Name	Composition	Grain Size	Color	Other Characteristics
Granite	Quartz, Feldspar, Mica or Amphibole	Coarse	White, Pink, Gray	Color and grain size highly variable
Rhyolite	Quartz, Feldspar	Very fine to glassy	Light color	Very high silica content, may contain larger crystals (phenocrysts)
Granodiorite	Quartz, Plagioclase	Coarse	Light gray, White & Black	High silica content, more plagioclase than alkali feldspar
Dacite	Quartz, Plagioclase	Fine	Light gray, White	High silica content, may contain larger crystals (phenocrysts)
Diorite	Plagioclase, Biotite, Hornblende, Pyroxene	Coarse	White and dark gray	Fairly rare, often confused with gabbro or granodiorite
Basalt	Augite, Pyroxene, Plagioclase, low-silica	Fine	Dark gray, sometimes light	Often has gas bubbles (vesicles). Can contain some larger crystals (porphyritic)
Diabase	Plagioclase, Augite, Olivine	Fine	Gray to dark gray	Sometimes called ‘microgabbro’. Fine but visible crystals
Gabbro	Augite, Pyroxene, Plagioclase, low-silica	Coarse	Gray to dark gray	Intrusive equivalent of basalt
Pumice	Highly varied	Very fine, glassy	Light	Many small gas bubbles (vesicles), floats in water
Scoria	Augite, Pyroxene, Plagioclase, low-silica	Very fine	Dark	Larger gas bubbles (vesicles) and vesicle walls than pumice. Usually does not float.
Obsidian	Quartz, feldspar	Glassy	Dark, light (rarely)	Conchoidal fractures, very sharp edges
Andesite	Plagioclase, Pyroxene, Hornblende	Fine grained, som large	Light to dark gray	Between basalt and rhyolite. Porphyritic texture (some large crystals)
Tuff	Highly variable	Very fine	Light to dark	Relatively soft, made of volcanic ash
Pegmatite	Quartz, Feldspar, Mica	Coarse to Very large	White, Pink, Gray, Varied	Resembles granite, but even bigger crystals
Syenite	Alkali Feldspar	Coarse	Light to medium	Similar to granite but lacking quartz
Dunite	Olivine, Pyroxene	Coarse	Green	Fairly rare. Almost entirely olivine crystals

Metamorphic

Metamorphic rocks are formed when existing rocks are subjected to heat and pressure, changing their shape, appearance, and composition. They are often distinguishable from other rock types by their foliation and the presence of characteristic metamorphic minerals like garnet, kyanite, and staurolite.

You can find my complete post about metamorphic rock identification here.

The type of metamorphic rock that is created depends on the original rock (known as the protolith) and the temperatures and pressures to which it has been subjected. Some rocks undergo multiple stages of metamorphism.

Rock Type	Composition	Grain Size	Foliation	Color	Other Characteristics
Gneiss	Quartz, Feldspar, Mica, Varied	Coarse	Yes	Light and dark	Alternating bands of light and dark
Schist	Mica, Chlorite, Quartz, Feldspar, Varied	Medium to Coarse	Yes	Light to dark	Platy, flaky texture. Defined by texture, so composition is highly variable
Marble	Calcite, Dolomite	Fine to coarse	No	Usually white, with swirls of other colors	Soft, metamorphosed limestone or dolostone
Quartzite	Quartz	Medium	No	White, Gray, Pink	Hard, metamorphose sandstone
Slate	Clay or volcanic ash, Quartz	Very Fine	Yes	Gray	Sometimes confused for shale, breaks into sheets
Phyllite	Mica, Chlorite, Quartz	Fine	Yes	Gray, Green	Silky luster, breaks into sheets
Serpentinite	Serpentine, Brucite, Magnetite	Fine	No	Light to dark green	Greasy feeling
Amphibolite	Hornblende, Actinolite, Plagioclase	Medium to coarse	Yes, weak	Dark	Dense, very little silica
Granulite	Feldspar, Quartz, etc.	Medium to coarse	No	Light	Red or pink garnet crystals common
Hornfels	Varied -Clays, Quartz, Feldspars, Hornblende, Calcite	Fine	No	Varied from light, dark, green, etc.	Very hard, originally sandstone
Skarn	Calcite, Dolomite, Garnet, Pyroxene	Coarse	No	Light to medium	Very hard, can form from sedimentary or igneous protolith
Soapstone	Talc, Chlorite, Amphibole	Fine	No	Light	Schistose or massive texture. Very soft, may feel like soap
Migmatite	Varied – at least two distinct rock types	Fine to medium	No	Light and dark	Clearly visible layers that may appear ‘melted’

Utilize Available Rock Identification Resources

Here are some of the most popular rock identification tools available:

• Smithsonian Handbooks: Rocks & Minerals – This book is the most useful I have found for help in identifying rocks and minerals. It has gorgeous pictures and nice descriptions of hundreds of rocks and minerals. The only drawback to a book like this is that it can sometimes be difficult to sift through to find a rock that matches yours. Even if you have gone through the process of documenting your rock’s physical properties, it can take a while to match it with a rock type in the book.
• The Rock Key – This tool is a little dated but the concept and information are great. Using many of the physical properties I listed above, you just answer questions about your rock and it moves through a flow chart until you land on an answer. It’s a little simplistic and doesn’t cover every type of rock, but for most major rock types it works very well.
• Stone ID App (Google Play) – This app uses AI and image recognition algorithms to identify what kind of rock you have. You can also find it on the Apple store here. I have found that this app (and others like it) is unreliable, but your mileage may vary. They rely on sophisticated algorithms that compare an image of your rock (taken from your phone) to thousands of others. Sometimes it works, and sometimes it doesn’t. If nothing else, it’s fun to play around with!
• What Is The Rock? Subreddit – This is a place dedicated to trying to figure out what kind of rock you have. It is often difficult to tell what a rock is based solely on pictures, so the more information you can provide (like the physical properties I listed above) the better. Take nice, well-lit pictures from multiple angles, and then make a post on the subreddit. Users (maybe even me!) can then discuss and make the best guess about what kind of rock you have.