DIAMOND PROPERTIES & TYPES - Diamond is exponentially harder than corundum. Diamond is also the least compressible and stiffest substance. Diamond is an exceptional thermal conductor - 4 times better than copper - which gives significance to diamonds being called 'ice'. Diamond has an extremely low thermal expansion, is chemically inert with respect to most acids and alkalis, is transparent from the far infrared through the deep ultraviolet, and is one of only a few materials with a negative work function (electron affinity). Diamonds do not conduct electricity well, although some are semiconductors. Diamond can burn if subjected to a high temperature in the presence of oxygen. The brilliance and fire of a diamond are due to its high dispersion and high refractive index.

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Diamond

Properties & Types

by Anne Marie Helmenstine, Ph.D. 

Diamond is the hardest natural material.

The Mohs hardness scale, on which diamond is a '10' and corundum (sapphire) is a '9', doesn't adequately attest to this incredible hardness, as diamond is exponentially harder than corundum. 

Diamond is also the least compressible and stiffest substance.

Diamond is an exceptional thermal conductor - 4 times better than copper - which gives significance to diamonds being called 'ice'.

Diamond has an extremely low thermal expansion, is chemically inert with respect to most acids and alkalis, is transparent from the far infrared through the deep ultraviolet, and is one of only a few materials with a negative work function (electron affinity).

One consequence of the negative electron affinity is that diamonds repel water, but readily accept hydrocarbons such as wax or grease.

Diamonds do not conduct electricity well, although some are semiconductors.

Diamond can burn if subjected to a high temperature in the presence of oxygen.

Diamond has a high specific gravity; it is amazingly dense given the low atomic weight of carbon.

The brilliance and fire of a diamond are due to its high dispersion and high refractive index.

Diamond has the highest reflectance and index of refraction of any transparent substances.

Diamond gemstones are commonly clear or pale blue, but colored diamonds, called 'fancies,' have been found in all the colors of the rainbow. 

Boron, which lends a bluish color, and nitrogen, which adds a yellow cast, are common trace impurities.

Two volcanic rocks that may contain diamonds are kimberlite and lamproite.

Diamond crystals frequently contain inclusions of other minerals, such as garnet or chromite.

Many diamonds fluoresce blue to violet, sometimes strongly enough to be seen in daylight.

Some blue-fluorescing diamonds phosphoresce yellow (glow in the dark in an afterglow reaction).

Type of Diamonds

Natural Diamonds

Natural diamonds are classified by the type and quantity of impurities found within them.

·      Type Ia - This is the most common type of natural diamond, containing up to 0.3% nitrogen.

·      Type Ib - Very few natural diamonds are this type (~0.1%), but nearly all synthetic industrial diamonds are. Type Ib diamonds contain up to 500 ppm nitrogen.

·      Type IIa - This type is very rare in nature. Type IIa diamonds contain so little nitrogen that it isn't readily detected using infrared or ultraviolet absorption methods.

·     Type IIb - This type is also very rare in nature. Type IIb diamonds contain so little nitrogen (even lower than type IIa) that the crystal is a p-type semiconductor.

Synthetic Industrial Diamonds

Synthetic industrial diamonds have produced the process of High-Pressure High-Temperature Synthesis (HPHT).

In HPHT synthesis, graphite and a metallic catalyst are placed in a hydraulic press under high temperatures and pressures.

Over the period of a few hours, the graphite converts to diamond.

The resulting diamonds are usually a few millimeters in size and too flawed for use as gemstones, but they are extremely useful as edges on cutting tools and drill bits and for being compressed to generate very high pressures.

(Interesting side note: Although used to cut, grind, and polish many materials, diamonds aren't used to machine alloys of iron because the diamond abrades very quickly, due to a high-temperature reaction between iron and carbon.)

Thin Film Diamonds

A process called Chemical Vapor Deposition (CVD) may be used to deposit thin films of polycrystalline diamond.

CVD technology makes it possible to put 'zero-wear' coatings on machine parts, use diamond coatings to draw the heat away from electronic components, fashion windows that are transparent over a broad wavelength range, and take advantage of other properties of diamonds.

Anne Marie Helmenstine, Ph.D.

Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

ThoughtCo and About Education chemistry expert since 2001.

Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

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https://www.thoughtco.com/diamond-properties-and-types-602111