How to Identify a Rock

Gneiss.  A banded foliated metamorphic rock with coarse grain.

As with anything, such as plants, birds, or animals, rocks can be quite difficult to identify.  There are however, far fewer rocks to identify world wide than there are living organisms.  At a local scale there may be only one or two major rocks that need to be identified while there may be several dozen species of plants or birds.  On top of that, there is likely whole lot of rocks of the same type at a particular location which helps aid identification.  Plants of particular species may be few and far between while birds and animals fly or run away before you can ever get a good look.  So rocks may be one of the easiest things to identify in any particular habitat.  That doesn't always mean they are easy to identify though.  It is fairly typical for someone to just flip through page after page of a field guide trying to identify a rock, bird, or plant.  Then, after flipping through the entire guide realizing you couldn't find what you were looking for.  Likely the species is in the guide but you just didn't know what you were looking for.  There are several things you can do to help narrow down your search for a rock, these include color, texture, and structure.

First color.  Is the rock light, medium, or dark colored?  Of course these are generalizations and the rock might be pinkish or tan or brown.  But, even if it is pinkish it is likely a light colored rock such as granite, quartzite, or possibly rhyolite.  The rock may also have both dark and light specks through out it.  If the rocks are sort of an even "salt and pepper", with approximately even amounts of light and dark, it would be considered medium in color, such as diorite.  Even light colored rocks will have some dark specks in them or dark colored rocks some light colored specks.  The key is determining the overall generalized color of the rock. 

Granite.  A coarse, light colored igneous rock.

Secondly consider rock texture.  Is it fine grained where you can hardly see any crystals such as with the dark colored basalt or light colored rhyolite?  Is it coarse grained where you can easily see individual crystals such as with light colored granite or dark colored gabbro?  Other rocks may have lots of holes in them such as scoria or pumice.  These holes are a result of lava quickly cooling and forming air bubbles called vesicles. 

Third, what is the overall structure or pattern of the rock.  Does the rock show irregular banding patterns such as gneiss, or no pattern such as with granite?  Does the rock break into plates like with schist or slate?  Or are there many long parallel bands which are called bedding planes such as with sedimentary rocks like limestone. 

Limestone.  A light colored sedimentary rock that forms bedding planes.  No bedding planes visible in this picture. 

Also, narrowing a rock down to igneous, metamorphic or sedimentary can help a lot.  Igneous rocks are volcanic in origin and have interlocked crystals that range from coarse to fine.  The crystals have flat surfaced and fit together like a puzzle with no air spaces.  Typically there is not overall structure or pattern to igneous rocks.  Metamorphic rocks are formed by heat and pressure.  They also are made of crystals that interlock and have flat surfaces.  They do however form banding patterns of light to dark, or break into plate like pieces.  This banded or plate like structure is called foliation.  Not all metamorphic rocks are foliated such as with marble and quartzite.  Sedimentary rocks do not have interlocking crystals.  They often contain fossils and form bedding planes.  Bedding planes are thin parallel bands.Sedimentary rocks are formed through the accumulation of sediments such as sand or clay that turn into rocks.

Sandstone cliffs.  A coarse textured sedimentary rock made or sand.  Individual sand crystals do not interlock.  Sandstone forms bedding planes that run parallel to each other as seen in this picture.

By narrowing your choices down using color, texture, structure as well as igneous, metamorphic, and sedimentary, finding the right rock will be a lot easier.  Here is another great resource to help identify rocks more specifically than I have done here:

http://geology.about.com/od/rocks/a/Rock-Tables.htm