About Granite

Granite makes mountains, much of the earth that we walk on and--not least--countertops. Abundant, strong and often beautiful, granite has always been a valuable resource. When the pharaohs wanted to make an impression, which was often, they used granite. The use of granite today reflects a similar, if less grandiose, desire: to make something beautiful that will last for a very long time.

Identification

Granite is one of Earth's most abundant rocks, and constitutes most of the continental crust. The word, from the Latin "granus," or "grainy," describes its appearance and texture: composed primarily of feldspar with elements of quartz, along with smaller quantities of minerals such as hornblende and mica, granite is speckled with evenly distributed crystals. The size of the crystals determines whether granite feels smooth or rough. It is a very durable stone, largely because of its tight internal structure and its component of quartz.

Types

Depending on the exact combination of the minerals within granite, its color can range from very light to a dark, almost black, gray. The color of the constituent minerals determines the color of the granite. Quartz, for instance, occurs in a range of shades from yellowish-white to pink; a high proportion of quartz will produce a light granite dominated by one of these colors. Darker granite may contain a concentration of hornblende or mica. Because the proportions of minerals vary from place to place, no two quarries will produce granite that looks precisely identical.

Geography

Granite occurs throughout the world, particularly near mountain ranges or areas that, millions of years ago, were mountain ranges. In these areas tectonic shifts heaved the granite up from the depths of the earth to become accessible. Scandinavia, the Iberian Peninsula, southern Africa, Brazil and India all participate heavily in the export of granite. In Barre, Vermont, is the world's largest deep granite quarry, while the largest open-faced granite quarry is in Mount Airy, North Carolina. It is the New Hampshire, however, that has the nickname of the Granite State, a reflection of its history as a primary source for the stone.

Function

The beauty and strength of granite has made it a favorite building stone. It often serves as a facing or veneer for less visually appealing materials: many of the Egyptian pyramids, for instance, were built of limestone but covered in granite. It is harder than marble and less susceptible to acids. For this reason, granite is replacing marble as a building stone in parts of the world where acid rain occurs. Similarly, builders and decorators prefer granite to marble countertops because granite better resists staining and pitting.

Misconceptions

Some controversy exists about the origins of granite. While it is usually classified as an igneous rock, one that resulted from the cooling of magma extruded from Earth's mantle, its wide distribution suggests an alternate source for most granite deposits. Shale and sandstone buried deep beneath the Earth's crust and exposed to great heat may have formed granite. If this is the case, granite could be considered a metamorphic rock. Because there are a great variety of granite types, multiple explanations for their origins are possible.

Resources

• A geology professor and granite aficionado covers the subject from top to bottom.