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The most popular gemstone.   The hardest known substance.   An amazing number of uses.

diamond crystal

Diamond crystal: A natural, uncut octahedral diamond crystal. © iStockphoto / Timo Klein.

What is Diamond?

Diamond is a rare, naturally-occurring mineral composed of carbon. Each carbon atom in a diamond is surrounded by four other carbon atoms and connected to them by strong covalent bonds. This simple, uniform, tightly-bonded arrangement yields one of the most durable substances known.

Diamond is a fascinating mineral. It is chemically resistant and it is the hardest known natural substance. These properties make it suitable for use as a cutting tool and for other uses where durability is required. Diamond also has special optical properties such as a high index of refraction, high dispersion, and high luster. These properties help make diamond the world's most popular gemstone.

Diamonds are a bit of a mystery. They are composed of the element carbon, and because of that many people believe that they must have formed from coal. Many teachers still teach this in their classrooms. But that is not true!

Physical Properties of Diamond

Chemical Classification Native element
Color Most industrial-grade diamond is black in color due to impurities. Gem diamond occurs in many colors, including colorless, yellow, red, orange, green, blue, and brown.
Streak Diamond is harder than a streak plate - when this occurs the streak is called "colorless"
Luster Adamantine - the highest level of luster for a nonmetallic mineral.
Diaphaneity Transparent
Cleavage Perfect, octahedral
Mohs Hardness 10 - the hardest mineral
Specific Gravity 3.4 to 3.6
Diagnostic Properties Hardness, heat conductivity, crystal form, index of refraction, dispersion
Chemical Composition C (elemental carbon)
Crystal System Isometric
Uses Gemstones, industrial abrasives

Diamond Consumption in the United States
In 2010, consumers in the United States spent about $19 billion on gemstones. Of that amount, $18 billion was spent on diamonds, and less than $1 billion was spent on colored stones. Diamonds are the most popular gemstones with U.S. consumers by a wide margin.

How Do Diamonds Form?

Diamonds are not native to Earth's surface. Instead they form at high temperatures and pressures that occur in Earth's mantle about 100 miles down.

How do diamonds form?
How do diamonds form?
A detailed article that explains the four sources of diamonds found at Earth's surface.

Most of the diamonds that have been discovered were delivered to Earth's surface by deep-source volcanic eruptions. These eruptions begin in the mantle, and on their way up they tear out pieces of mantle rock and deliver them to Earth's surface without melting. These blocks from the mantle are known as xenoliths. They contain diamonds that were formed at the high temperature and pressure conditions of the mantle.

People produce diamonds by mining the rock that contains the xenoliths or by mining the soils and sediments that formed as the diamond-bearing rock weathered away.

Some diamonds are thought to form in the high temperature-pressure conditions of subduction zones or asteroid impact sites. Some are delivered to Earth in meteorites. No commercial diamond mines have been developed in deposits with these origins.

World diamond production histogram

Leading diamond producers: This chart shows the estimated annual production of gem-quality diamonds, in millions of carats, for the world's leading diamond-producing nations. Graph by Geology.com. Data from USGS Mineral Commodity Summaries. Learn about the countries that produce diamonds.

Gem Diamonds vs. Industrial Diamonds

Gem diamonds are stones with color and clarity that make them suitable for jewelry or investment use. These stones are especially rare and make up a minor portion of worldwide diamond production. Gemstone diamonds are sold for their beauty and quality.

Natural diamond crystals have a specific gravity that ranges between approximately 3.4 to 3.6. This range exists because most diamonds contain impurities and have irregularities in their crystal structure. Gem quality diamonds are the most perfect diamonds, with minimal impurities and defects. They have a specific gravity that is very close to 3.52.

Industrial diamonds are mostly used in cutting, grinding, drilling, and polishing procedures. Here, hardness and heat conductivity characteristics are the qualities being purchased. Size and other measures of quality relevant to gemstones are not important. Industrial diamonds are often crushed to produce micron-sized abrasive powders. Large amounts of diamonds that are gemstone quality but too small to cut are sold into the industrial diamond trade.

dispersion demonstration

A demonstration of dispersion: White light being separated into its component colors while passing through a prism. Diamonds have a high dispersion. NASA Image.

Diamond as a Gemstone

Diamonds are the world's most popular gemstones. More money is spent on diamonds than on all other gemstones combined. Part of the reason for diamond's popularity is a result of its optical properties - or how it reacts with light. Other factors include fashion, custom, and marketing.

Diamonds have a very high luster. The high luster is a result of a diamond reflecting a high percentage of the light that strikes its surface. This high luster is what gives diamonds their pleasing "sparkle."

Diamond also has a high dispersion. As white light passes through a diamond, this high dispersion causes that light to separate into its component colors. Dispersion is what enables a prism to separate white light into the colors of the spectrum. This property of dispersion is what gives diamonds their colorful "fire."

diamond fire

Diamond fire: A round brilliant cut diamond showing "fire." © iStockphoto / Greg Stanfield.

fancy diamonds

Fancy diamonds: Small "fancy" diamonds in purple and canary yellow colors. Stones are about 4 millimeters across.

cubic diamond crystal

Cubic diamond crystal: A green diamond crystal. The color and cubic crystal shape are natural. Many natural diamond crystals are cubic or octahedral in shape. This diamond is about 4 millimeters across and is suitable for industrial use.

Diamond Gemstone Quality

The quality of a diamond gemstone is primarily determined by four factors: color, cut, clarity, and carats. A standardized method of assessing diamond quality was developed in the 1950s by the Gemological Institute of America and is known as "The 4Cs of Diamond Quality" [5].

Color: Most gem-quality diamonds range from colorless to yellow. The most highly regarded stones are those that are completely colorless. These are the ones sold for the highest prices. However, another category of diamond gemstone is increasing in popularity. These are the "fancy" diamonds, which occur in a variety of colors including red, pink, yellow, purple, blue, and green. The value of these stones is based upon their color intensity, rarity, and popularity.

Cut: The quality of workmanship in a diamond has a large impact upon its quality. This influences not only the geometric appearance of the stone but also the stone's luster and fire. Ideal stones are perfectly polished to be highly reflective and emit a maximum amount of fire. The faceted faces are equal in size and identical in shape. And, the edges of each faceted face meet perfectly with each of its neighbors.

Clarity: The ideal diamond is free from internal flaws and inclusions (particles of foreign material within the stone). These detract from the appearance of the stone and interfere with the passage of light through the stone. When present in large numbers or sizes, they can also reduce the strength of the stone.

Carat: Diamonds are sold by the carat (a unit of weight equal to 1/5th of a gram or 1/142nd of an ounce). Small diamonds cost less per carat than larger stones of equal quality. This is because very small stones are very common and large stones are especially rare.

industrial diamonds

Industrial diamonds: Small diamonds, less then 1 millimeter in size, that are suitable for industrial use as abrasive granules. These diamonds might be polycrystalline, have numerous inclusions, have poor clarity, contain fractures, or have some other characteristic that disqualifies them from higher levels of use.

diamond oil well drill bit

Diamond drill bit: A drill bit used in the drilling of oil wells. Each of the cutting tips has small grains of diamond embedded in the metal. These cut their way through the rock as the bit turns. © iStockphoto / mikeuk.

diamond concrete saw

Diamond concrete saw: A concrete saw with a diamond blade of about four feet diameter. US Air Force Image.

Diamonds Used as an Abrasive

Because diamonds are very hard (ten on the Mohs scale) they are often used as an abrasive. Most industrial diamonds are used for these purposes. Small particles of diamond are embedded in a saw blade, a drill bit or a grinding wheel for the purpose of cutting, drilling or grinding. They might also be ground into a powder and made into a diamond paste that is used for polishing or for very fine grinding.

There is a very large market for industrial diamonds. Demand for them exceeds the supply obtained through mining. Synthetic diamonds are being produced to meet this industrial demand. They can be produced at a low cost per carat and perform well in industrial use.

Mineral collection

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.

Other Uses of Diamonds

Most industrial diamonds are used as abrasives. However, small amounts of diamond are used in other applications.

Diamond windows

  are made from thin diamond membranes and are used to cover openings in lasers, x-ray machines, and vacuum chambers. They are transparent, very durable, and resistant to heat and abrasion.

Diamond speaker domes

  enhance the performance of high-quality speakers. Diamond is a very stiff material, and when made into a thin dome it can vibrate rapidly without the deformation that would degrade sound quality.

Heat sinks

  are materials that absorb or transmit excess heat. Diamond has the highest thermal conductivity of any material. It is used to conduct heat away from the heat-sensitive parts of high-performance microelectronics.

Low-friction microbearings

  are needed in tiny mechanical devices. Just as some watches have jewel bearings in their movements, diamonds are used where extreme abrasion resistance and durability are needed.

Wear-resistant parts

  can be produced by coating surfaces with a thin coating of diamond. In this process, diamond is converted into a vapor that deposits on the surface of parts prone to wear.

synthetic diamonds

Synthetic diamonds of various colors grown by the high-pressure high-temperature technique. Image by Wikipedia contributor Materialscientist.

Diamond Information
[1] Gemstones: Donald W. Olson, U.S. Geological Survey, Mineral Commodity Summaries, January 2016.

[2] Gemstones: Donald W. Olson, U.S. Geological Survey, 2013 Minerals Yearbook, March 2016.

[3] Diamond, Industrial: Donald W. Olson, U.S. Geological Survey, 2016 Mineral Commodity Summaries, January 2016.

[4] Diamond, Industrial: Donald W. Olson, U.S. Geological Survey, 2013 Minerals Yearbook, December 2015.

[5] Diamond Quality Factors: An explanation of how diamonds are evaluated using the 4Cs. Gemological Institute of America. Last accessed July 2016.

Synthetic Diamonds and Simulants

Synthetic diamond
Flawless Synthetic Diamonds by Chemical Vapor Deposition

Diamond is a very valuable material, and many people have worked to create synthetic diamonds and diamond simulants. Synthetic diamonds are man-made materials that have the same chemical composition, crystal structure, and properties as natural diamonds. Diamond simulants are materials that look like diamonds but have different chemical compositions and physical properties.

The first commercially successful synthesis of diamond was accomplished in 1954 by workers at General Electric. Since then, many companies have been successful at producing synthetic diamond suitable for industrial use. Today, most of the industrial diamond consumed is synthetic, with China being the world leader with a production of over 4 billion carats per year.

In the last decade, a few companies have developed technology that enables them to produce gem-quality laboratory-created diamond up to a few carats in size in several colors. Some companies are using high-pressure, high-temperature methods, while others are using chemical vapor deposition methods. Their stones are being sold in stores and on the internet at a significant discount to natural stones of similar quality and size. These stones are required to be sold with a disclosure that they are "synthetic" or "laboratory-created."


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