Composition

Diamond is carbon in its most concentrated form. Except for trace impurities like boron and nitrogen, diamond is composed solely of carbon, the chemical element that is fundamental to all life.

  • Atomic Number: 6

  • Atomic Weight: 12.011

  • Outermost Electron: 4

But the diamond is distinctly different from its close cousins the common mineral graphite and lonsdaleite, both of which are also composed of carbon. why is diamond the hardest surface known while graphite is exceedingly soft? Why is diamond transparent while graphite is opaque and metallic black? What is it that makes diamond so unique?

The key to these question lie in diamond's particular arrangement of carbon atoms or its crystal structure.

Crystal Structure

A neutral carbon atom has 6 protons and 6 electrons surrounding its nucleus. Four of the electrons in a carbon atom are valence electrons, which are electrons that are available to form bonds with other atoms. In graphite, each carbon atom bonds only 3 of its 4 valence electrons with neighboring carbons. The resulting structure of these bonds is a flat sheet of connected carbon atoms. Though individually strong, these layers are only weakly connected to one another, and the ease with which they are separated is what makes graphite so slippery.


In diamond, however, every carbon shares all 4 of its available electrons with adjacent carbon atoms, forming a tetrahedral unit. This shared electron-pair bonding forms the strongest known chemical linkage, the covalent bond, which is responsible for many of diamond's superlative properties. The repeating structural unit of diamond consists of 8 atoms which are fundamentally arranged in a cube.

The diamond structure shows how each atom(red ball) is connected to 4 other carbon atoms by strong chemical bonds, creating diamond's rigid crystal structure.

Crystal Habits

Using this cubic form and its highly symmetrical arrangement of atoms, diamond crystals can develop in a variety of different shapes known as "crystal habits." The octahedron or eight-sided shape that we associate with diamonds is its most common crystal habit. But diamond crystals can also form cubes, dodecahedra, and even combinations of these shapes. All of these shapes are manifestations of the cubic crystal system to which the mineral diamond belongs. Two exceptions are the flat form called a macle, which is actually a composite crystal, and etched crystals, which have rounded surfaces and, sometimes, elongated shapes.

Cube

Octahedron

Cubo-octahedron

Dodecahedron

Macle twin

Diamond Crystal

​​Real diamond crystals don't have completely smooth faces. Trigons are triangular growths that reflect subtle changes in height on a diamond's face. The trigons shown here are slight indentations that were most likely produced by a natural etching of the crystal. However, raised trigons, which point in the same direction as the crystal face, may also occur from etching, dissolution, or as part of the natural growth of the crystal.

Trigon Photo by ChrisMago

Hardness

​Diamond is renowned for its hardness. Hardness is the measure of a substance's resistance to being scratched, and only a diamond can scratch another diamond. Diamond is the hardest substance known.

The Mohs scale--a hardness scale developed in 1822 by Austrian Friedreich Mohs as a criterion for mineral identification -- can help us appreciate the hardness of diamond. The scale ranks 10 minerals; harder minerals, with a higher number, can scratch those with a lower number.

 When the mineral hardness numbers from the Mohs scale are plotted against those on the more quantitative  Knoop scale (based on the force needed to make indentations using a diamond), we can see how it doesn't adequately express the extreme hardness of diamond. The Mohs scale is relatively stable until it reaches the eighth mineral topaz, but it jumps exponentially from corundum (colorless sapphire) to diamond. It is in fact difficult to measure the hardness of diamond, because diamond must be used to measure its own hardness.

Density

Density is a ratio of a substance's mass to its volume. For instance, density explains why a certain amount of lead feels heavier than an equal volume of salt. Diamond is amazingly dense given the low atomic weight of carbon. At 3.51 grams per cubic centimeter, diamond is much more dense than graphite, which weighs in at only 2.20 grams per cubic centimeter. This comparison offers an important clue to diamond's origin: the fact that diamond's carbon atoms are "squeezed" together tighter than in graphite, which forms near Earth's surface, implies that diamond is formed under high pressure conditions. This concept was  corroborated by the experimental synthesis of diamond at high pressure and temperature illustrated on the graph below.

The conditions of

Diamond Formation

This simplified diagram shows the conditions of pressure and temperature where diamond and graphite will be the stable forms of carbon. The points show the conditions at which diamonds were first grown by the companies ASEA and General Electric in the early 1950s. Temperatures are in Kelvin--subtract 273 to convert to degrees Celsius.

This magnitude of pressure is difficult to comprehend. For example, the pressure of 55,000 atmospheres necessary to make a diamond at 1400 degrees C (orange hot) would require: the Eiffel Tower (7000 metric tons) resting on a 5 inch plate.

Fluorescence and

Phosphorescence

An interesting property of some diamonds is that they can glow in the dark. When illuminated by ultraviolet light, certain diamonds can absorb the high-energy radiation and re-emit it as visible light. These diamonds are called fluorescent. Some can even continue glowing after the ultraviolet source is turned off. These diamonds are phosphorescent.

All diamonds do not fluoresce, only about 30% of diamonds exhibit some degree of fluorescence. Most diamond grading laboratories consider diamond fluorescence an identifying characteristic but it is not a grading factor 4Cs. Just report and describe a diamond's fluorescence by its intensity as like 'None, Faint, Medium,  Strong and Very Strong. Some experts of diamond think blue fluorescence enhances a diamond's color, especially diamond whit I to  M color grades. This blue fluorescence can make a faint yellowish diamond with a very strong to medium bluish fluorescence may have a slightly higher color grade than similar diamonds that do not fluoresce.

Fluorescence in Rough and Polished Diamond Photo by ChrisMago 

Thermal

Conductivity

Diamonds are called "ice" with good reason. Objects feel cold not only because they are at a

lower temperature than our bodies, but also because they can extract or conduct the heat away from us. When you touch a diamond to your lips, it feels ice-cold because it robs your lips of their heat. The capacity of diamond to conduct heat distinguishes it readily from other gems and exceeds that of copper, an excellent thermal conductor, by about 4 times at room temperature. This exceptional property of diamond is increasingly being used for extracting heat from electronic devices to make them smaller and more powerful.

Metals usually conduct heat much better than transparent substances, because they have loose electrons that act as packets for carrying heat in much the same way they move electricity.

Nonmetals conduct heat solely by atomic vibrations, a less efficient mechanism than moving electrons. In diamond, however, vibrational energy travels through the crystal along the strong internal chemical bonds. Thus, diamond's superlative strength provides excellent thermal conduction as well.

What is diamond ?

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