Marble Rock: Geology, Properties, Uses

Marble is a crystalline metamorphic rock. In its pure form, it's white.
Marble is a crystalline metamorphic rock. In its pure form, it's white. Joelle Icard / Getty Images

Marble is a metamorphic rock formed when limestone is subjected to high pressure or heat. In its pure form, marble is a white stone with a crystalline and sugary appearance, consisting of calcium carbonate (CaCO3). Usually, marble contains other minerals, including quartz, graphite, pyrite, and iron oxides. These minerals can give marble a pink, brown, gray, green, or variegated coloration. While true marble forms from limestone, there is also dolomitic marble, which forms when dolomite [CaMg(CO3)2] undergoes metamorphosis.

How Marble Forms

Marble caves over General Carrera Lake, Puerto Tranquilo, Chile.
Marble caves over General Carrera Lake, Puerto Tranquilo, C. Enn Li  Photography / Getty Images

Limestone, the source material for marble, forms when calcium carbonate precipitates out of water or when organic debris (shells, coral, skeletons) accumulate. Marble forms when limestone experiences metamorphism. Usually, this happens at a convergent tectonic plate boundary, but some marble forms when hot magma heats limestone or dolomite. The heat or pressure recrystallizes calcite in the rock, changing its texture. Over time, the crystals grow and interlock to give the rock a characteristic sugary sparkling appearance.

Other minerals in marble also change during metamorphism. For example, clay recrystallizes to form mica and other silicates.

Marble is found all over the world, but four countries account for half of its production: Italy, China, Spain, and India. Probably the most famous white marble comes from Carrara in Italy. Carrara marble was used by Michelangelo, Donatello, and Canova for their masterpiece sculptures.

Properties

The visible crystals in marble give it a characteristic granular surface and appearance, but there are other properties used to identify the rock.

Marble is considered to be a strong, hard stone, even though its primary mineral, calcite, only has a Mohs hardness of 3. Marble can be scratched with a metal blade.

Marble tends to be light in color. The purest marble is white. Marble that contain a lot of bituminous material may be black. Most marble is pale gray, pink, brown, green, yellow, or blue.

Marble fizzes upon contact with dilute hydrochloric acid.

Uses

The statue of Abraham Lincoln in the Lincoln Memorial is made of white marble.
The statue of Abraham Lincoln in the Lincoln Memorial is made of white marble. Photo by Mike Kline (notkalvin) / Getty Images

Because of the way marble forms, it occurs in large deposits worldwide. It's economical to mine this common, useful rock on a large scale.

Most marble is used in the construction industry. Crushed marble is used to build roads, foundations of buildings, and railroad beds. Dimension stone is made by cutting marble into blocks or sheets. Dimension stone is used to make buildings, sculptures, paving stones, and monuments. The statue of Lincoln in the Lincoln Memorial is made of white marble form Georgia, while the floor is pink Tennessee marble and the exterior facade is marble from Colorado. Marble is susceptible to acid rain and weathering, so it wears down over time.

White marble is ground to make "whiting," a powder used as a brightener and pigment. Powdered marble, along with limestone, may be used as a calcium supplement for livestock. Crushed or powdered marble is used in the chemical industry to neutralize acid, as a pill filler, and to remediate acid damage in water and soil.

Marble may be heated to drive off carbon dioxide, leaving calcium oxide or lime. Lime is used in agriculture to reduce the acidity of soil.

The Other Definition of Marble

Sometimes polished travertine is called marble. Travertine is a sedimentary rock.
Sometimes polished travertine is called marble. Travertine is a sedimentary rock. lacer / Getty Images

In the stone trade and common usage, any crystalline carbonate that takes a high polish might be called "marble." Sometimes limestone, travertine, serpentine (a silicate), and breccia are called marble. Geologists use the narrow definition of a metamorphic rock formed from limestone or dolomite.

Are Marbles Made of Marble?

The original "marbles" were made of glass, not marble.
The original "marbles" were made of glass, not marble. Solveig Faust / EyeEm / Getty Images

The original toy called "marbles" bears the mark "Made in Germany." These playthings were made by rolling clay or another pottery material into balls, then glazing and firing it so that it resembled imitation agate. The marbles featured round "eyes" from the firing process, giving them a sort of marbled appearance.

Glass marbles entered mass production in 1846, with the German invention of marble scissors. Toys resembling marbles have been found in excavations of ancient Egyptian and Mesopotamian sites. Early marbles were rounded stones, nuts, or clay. While a few marbles are indeed made of marble, the stone is too soft to be an ideal material for the modern game. The name of the toy reflects the appearance of the balls, not their composition.

Key Points

  • Marble is a metamorphic stone formed by subjected limestone to heat or pressure.
  • In pure form, marble consists of calcium carbonate (calcite) and is sparkling white. Impurities produce pale gray, brown, or variegated colored rock. Black marble also occurs.
  • Marble takes a high polish. In common usage, any stone that takes a high polish may be called marble, but this is technically incorrect.
  • Marbles aren't made of marble. The toy got its name from its appearance rather than its composition. Ancient toys resembling marbles were made of smooth stone, clay, or nuts.

Sources

  • Acton, Johnny; Adams, Tania; Packer, Matt (2006). Origin of Everyday Things. Barnes and Noble.
  • Baumann, Paul (15 December 2004). "Collecting Antique Marbles: Identification and Price Guide". Krause Publications. Retrieved June 5, 2018.
  • Kearey, Philip (2001). Dictionary of Geology, Penguin Group, London and New York, p. 163.