Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth’s surface. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles and other minerals. This mineral composition usually gives granite a red, pink, gray or white color with dark mineral grains visible throughout the rock.
The Best-Known Igneous Rock
Granite is the best-known igneous rock. Many people recognize granite because it is the most common igneous rock
found at Earth's surface and because granite is used to make many objects that we encounter in daily life. These
include counter tops, floor tiles, paving stone, curbing, stair treads, building veneer and cemetery monuments.
Granite is used all around us - expecially if you live in a city.
Granite is also well-known from its many world-famous natural exposures. These include: Stone Mountain, Georgia;
Yosemite Valley, California, Mount Rushmore, South Dakota; Pike's Peak, Colorado; and White Mountains, New Hampshire.
The word “granite” is used in a variety of ways by different people. A simple definition is used in introductory
courses; a more precise definition is used by petrologists (geologists who specialize in the study of rocks); and,
the definition of granite expands wildly when used by people who sell decorative stone such as countertops, tile
and building veneer.
These multiple definitions of granite can lead to communication problems. However, if you know who is using the
word and who they are communicating with, you can interpret the word in its proper context. Three common usages of
the word “granite” are explained below.
A) Introductory Course Definition
Granite is a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz with minor amounts of
mica and amphibole minerals. This simple definition enables students to easily identify the rock based upon a visual inspection.
B) Petrologist's Definition
Granite is a plutonic rock in which quartz makes up between 10 and 50 percent of the felsic components and alkali feldspar accounts
for 65 to 90 percent of the total feldspar content. Applying this definition requires the mineral identification and quantification
abilities of a competent geologist.
Many rocks identified as “granite” using the introductory course definition will not be called “granite” by
the petrologist - they might instead be alkali granites, granodiorites, pegmatites or aplites. A petrologist might call these
“granitoid rocks” rather than granites. There are other definitions of granite based upon mineral composition.
This chart illustrates the generalized mineral composition of igneous rocks. Granites and rhyolites (compositionally equivalent to granite but of a fine grain size) are composed mainly of orthoclase feldspar, quartz, plagioclase feldspar, mica and amphibole.
C) Commercial Definition
The word “granite” is used by people who sell and purchase cut stone for structural and decorative use.
These "granites" are used to make countertops, floor tiles, curbing, building veneer, monuments
and many other products.
All of the rocks above would be called "granite" in the commercial stone industry. Clockwise from top left they are: granite, gneiss, pegmatite and labradorite. Click on any of the four samples above for an enlarged view. Each of the images above represents a slab of polished rock about eight inches across.
In the commercial stone industry a “granite” is a rock with visible grains that is harder than a marble.
Under this definition gabbro, basalt, pegmatite, schist, gneiss, syenite, monzonite, anorthosite, grannodiorite,
diabase, diorite and many other rocks will be called “granite”. The collection of images at right illustrates
the range of rocks that might be called "granite".
Uses of Granite
Granite is the rock most often quarried as a "dimension stone" (a natural rock material
that has been cut into blocks or slabs of specific length, width and thickness). Granite is
hard enough to resist most abrasion, strong enough to bear significant weight, inert enough
to resist weathering and it accepts a brilliant polish. These characteristics make it a
very desirable and useful dimension stone.
Most of the granite dimension stone produced in the United States comes from high quality deposits in
five states: Massachusetts, Georgia, New Hampshire, South Dakota and Idaho.
Granite has been used for thousands of years in both interior and exterior applications.
Rough-cut and polished granite is used in buildings, bridges, paving, monuments and many other
exterior projects. Indoors, polished granite slabs and tiles are used in countertops, tile
floors, stair treads and many other practical and decorative features.
High price often reduces the popularity of a construction material and granite often costs
significantly more than man-made materials in most projects. However, granite is frequenly
selected because it is a prestige material, used in projects to produce impressions of elegance,
durability and lasting quality.
Granite is also used as a crushed stone or aggregate. In this form it is used as a base material
at construction sites, as an aggregate in road construction, railroad ballast, foundations and
anywhere that a crushed stone is useful as fill.
Granite in the Continental Crust
Most introductory geology textbooks report that granite is the most abundant rock in the
continental crust. At the surface granite is exposed in the cores of many mountain ranges,
within large areas known as "batholiths," and in the core areas of continents known as "shields."
The large mineral crystals in granite are evidence that it cooled slowly from molten rock material.
That slow cooling had to have occurred beneath Earth's surface and required a long period of time
to occur. If they are today exposed at the surface the only way that could happen is if the granite
rocks were uplifted and the overlying sedimentary rocks were eroded.
In areas where Earth's surface is covered with sedimentary rocks, granites, metamorphosed granites
or closely related rocks are usually present beneath the sedimentary cover. These deep granites are
known as "basement rocks".
Rhodochrosite : a manganese mineral used as an ore, a pink gem and an ornamental stone.
Granite: The specimen above is a typical granite. It is about two inches across. The grain size is coarse enough to allow recognition of the major minerals. The pink grains are orthoclase feldspar and the clear to smoky grains are quartz or muscovite. The black grains can be biotite or hornblende. Numerous other minerals can be present in granite.
This video examines some of the granites that create the scenery and climbing pleasures of Yosemite National Park.
Photograph of a white, fine-grained granite. This specimen is about two inches across.
Magnified view of the white, fine-grained granite from the photograph above. The area shown in this image is about 1/4 inch across.
Photograph of a granite with very large crystals of orthoclase feldspar. Granites with such large crystals are known as "pegmatites". This rock is about two inches across.
This triangular diagram is a classification method for grantic rocks. It is based upon the relative abundance of feldspars (K-Na-Ca) and quartz. Mafic elements are not considered. It is modified after a classification chart prepared by the International Union of Geological Sciences. Image and modification by the United States Geological Survey.