Diamond - the supermaterial gemstone. The awesome physical properties of diamond.

By Julia Griffith FGA DGA EG

What's in this article?

Diamond properties - Hardness - Thermal conductivity - Thermal expansion - Low friction coeffiecient - Transparency - Mechanical strength - Sound propagation - Chemical stability - Why synthetic diamond is mainly used in industrial and scientific applications

Diamonds have fascinated us for millennia. The unparalleled lustre and ability to disperse and return light are just a few properties that contribute to the scintillating visual effects of diamond.

However, it is superior physical properties that have placed diamond at the top of the gemstone hierarchy, and is the reason why diamond is being hailed as "the material of the future".

This article celebrates diamond as a material and will discuss its most incredible properties relevant to industrial and scientific applications. Be prepared to be amazed as we look into the desirable properties of diamond....

Supermaterial definition: A material with remarkable physical properties (sometimes written as "supermaterial")

Diamond is "the best" for several desirable properties that far surpass the capabilities of other materials. This qualifies diamond as a Supermaterial.

Diamond is completely made out of carbon (with the exception of minor impurity atoms typically account for less than 0.2%). The carbon atoms in diamond are bonded together with the strongest bonds possible - covalent bonds. Carbon is a very small atom, therefore this simple chemical composition and repeating crystal structure results in an extremely dense material. In fact, diamond has the greatest atomic density our of all materials known to man (this means it has more atoms in any given space) and it it this dense crystal structure is the cause of many of diamonds impressive properties.

Here are some of diamonds most remarkable (non-cosmetic*) properties:

Hardness: Diamond is the hardest natural material known.

The most celebrated supermaterial property of diamond is its extreme hardness. This is what gained diamond the title of "the ultimate gemstone", and also why the famous DeBeers ad campaign "a diamond is forever" resonates so strongly - as it is, for the most part, true.

Hardness refers to a materials resistance to scratching and abrasion and diamond is the hardest natural material in existence*.

* I would like to say that diamond is the hardest material period - as prefixing it with the word "natural" almost undermines how hard diamond is. I've spoken to people that are surprised to find out that diamonds are harder than concrete or steel as these are manufactured products it was assumed that these would be harder. Make no mistake - diamond ranks near the top out of all materials IN EXISTENCE with only a few exceptions. These include; 1. Wurtzite boron nitride, which has been calculated to be 18% harder than diamond. 2. Lonsdaleite (covalently bonded carbon in 3D hexagonal formation), which has no current use, however, indicates areas of meteorite impact. This is calculated to be 58% harder than diamond.

The Moh's Scale of Hardness

Diamonds have a hardness of 10 on the Moh's scale of hardness, which is equivalent to an absolute hardness of 1500. Ruby and sapphire (corundum) score a 9 on the Moh's scale and have an absolute hardness of 400. Considering that corundum is one of the hardest materials on the planet - it can be said that diamonds are exponentially harder than everything else you will encounter. You will certainly never hold anything harder than a diamond (unless you study graphene or lonsdaleite). Where nothing is harder than diamond in day-to-day situations - nothing can scratch diamond except another diamond.

It is this super material property that gave diamond its "unbreakable" reputation and why diamonds are best suited for engagement rings compared with other gems. One can wear a diamond every day for a lifetime and, thanks to its hardness, it will look as perfect as the day it was cut. Need something that symbolises everlasting love? There truly isn’t a better material out there. DeBeers rode this all the way to the bank with their famous ad campaign "A diamond is forever" - a slogan embedded in truth.

Despite diamond's domination of the gemstone world, most natural diamonds are used as abrasives in manufacturing industries - not as gemstones in jewellery. Diamond's extreme hardness makes it an exceptionally useful abrasive. Diamond grit or powder can be coated or studded into metal, which can effortlessly cut through almost anything. Diamonds can effectively grind away non-ferrous metals and all other non-metals; however, ferrous metals (containing iron) cause an issue as the friction of diamond forms iron carbide resulting in excessive wear of the tools.

The high demand for industrial diamonds outweighed the natural supply. This is what spurred the creation of synthetic (laboratory-grown) diamonds, which were first successfully grown in the 1950s.

Want to learn all about lab-grown diamonds?

The ultimate online course on laboratory-grown diamonds

Thermal conductivity: The most effective thermal conductor known.

That's right! Diamond is the best thermal conductor out of all materials. Its thermal conductivity is 4 to 5x higher than copper. This is advantageous to a number of uses. In regards to abrasives - a diamonds hardness paired with it's super thermal conductivity means that diamonds can perform under the highest temperatures (caused by friction during cutting) without loosing performance or breaking.

They are also massively desirable in electronics. They act as incredible heat sinks - removing excess heat away from silicon circuits which helps to increase the performance of electronics. A good example of this is in phones. Diamonds help draw heat away from the circuit AND nano diamonds are impregnated into the glass offering a further large area for heat to escape from the device. Awesome, right?

Both of these uses utilise synthetic (laboratory-grown) diamonds. Synthetic diamonds grown by chemical vapour deposition are perfect for heat sinks as they can be grown in very thin layers. Technology made of synthetic diamond instead of silicon is also in our near future. This material can be grown to be electrically conductive (when doped with boron impurities) meaning diamond can act as the electrical conductor and heat sink all in one. The amount of synthetic diamond required is a fraction of the amount of silicon - resulting in smaller (yet more powerful) devices.

Thermal expansion: Lowest thermal expansion known.

Here's another reason that synthetic diamond is so good in electronics. All diamonds have very little thermal expansion, which means there is need to compensate for expansion and contraction of the material, which occurs with silicon.

A lack of thermal expansion is also one factor that makes diamonds so resilient as a gemstone. We could heat a diamond up and plunge it into liquid nitrogen with no negative effects. Not many materials can do this. As an example, the popular gemstone tanzanite has been known the fracture due to thermal contraction when leaving a warm house into a cold winter's night (we can assume this is in a cold area where temperatures are around freezing rather than somewhere like Florida).

Low friction coeffiecient: Extremely low surface friction.

Amazingly, diamond has a friction coefficient comparable to teflon - ranging from the same value to just a bit higher. Crazy, huh? This is not what one would expect from an abrasive. For this reason, synthetic diamonds can be cut and made into bearings. Think of synthetic ruby bearings that are used in automatic watches - diamonds make a more effective bearing and would last longer.

This property has interesting potential uses in medicine. Nano diamonds are being used to coat replacement joints, which last much longer than the pure-metal counterparts. Where diamond is biologically inert - it has no ill effects on the body.

Read more about lab-grown diamonds:

Laboratory-grown diamonds 101: An introductory guide to what, why and "how much?"

Chemical stability: Diamonds ar extremely inert.

Acids and alkalis, light, heat... diamonds have no reaction (with the exception of a few diamonds that have unstable colour that can be altered by light or heat, i.e. chameleon diamonds and some pink diamonds).

Diamonds are surprisingly resistent to radiation. Extreme energy (such as gamma rays) will change a diamonds colour as atoms are ejected from the crystal structure and these vacancies then absorb light; however, the overall structure remains sturdy and will last alot longer than other materials. For this reason, synthetic diamonds are being utilised in environments with high levels of radioactivity.

Mechanical strength: Diamonds can withstand pressures up to 1050GPa without breaking.

Diamonds are somewhat brittle if one smashes them with a hammer. However, they can take extreme pressures when applyied in a controlled manner. An interesting use of this is the invention of the diamond anvil, which essentially two diamonds that can be pushed together within a hydraulic press. These can be used to apply extreme pressures to materials that are placed between the two stones.

Transparency: Diamond is transparent to more wavelengths than any other material - making them extremely useful in laser technology (optics), allowing scientist to use wavelengths of light not used in other types of lasers.

Sound propagation: Sound travels faster in diamond than almost all other materials. (This isn't of much scientific use at this time but it's a pretty cool fact).

These are just a few of the amazing properties of diamonds... and I've mainly focused on the ones that make it a useful material in industry and science. It is understandable why diamonds are seen as the material of the future. This technology is really spurring on scientific research into diamond synthesis as, for the most part, synthetic diamonds are required.

Want to learn all about lab-grown diamonds?

The ultimate online course on laboratory-grown diamonds

Why are synthetic diamonds more appropriate for industrial and scientific uses?

There are two main reasons. Availability and quality.

Industrial diamond:

There just isn't enough natural diamond to fulfil the demand for diamond abrasives. Diamonds are rare, no matter who says otherwise - they are a rare material on this planet. They are not uncommon.... with approximately 130 million carats mined each year (please note: this figure is falling), which roughly provides about 100 million carats of industrial quality diamond. This is equal to 20 tonnes of industrial diamond from natural origins.

Synthetic diamond producers currently grow over 15 billion, yes - billion, carats of synthetic diamond per year for use as industrial abrasives. This means over 99% of diamond used for industrial uses is manmade and less than 1% is low-quality natural diamond. These synthetic diamonds are grown as small crystals suitable for diamond grit or crushing into dust.

Where are 15 billion carats of synthetic diamonds going every year? Mainly manufacturing. Although diamonds are forever, the metal that holds them in place is not. Drill bits and saws last longer and are more effective when impregnated with diamond; however, they still need replacing regularly.

Scientific uses:

The diamond required for heat sinks need to be of exceptional purity and quality. Such diamonds are exceptionally rare in nature, accounting for less than 2%. Even if we has a bigger supply of high purity natural diamonds - a lack of large sizes and the prevalence of diamond inclusions would still cause an issue. Above all of this - the high expense would deem the use of these natural high purity diamond uneconomic.

Synthetic diamonds can be engineered to better suit its intended use, which is a further advantage to using synthetic stones as their growth can be controlled. Examples include the doping the growth chamber with impurities, such as boron, silicon or nitrogen so that the diamond exhibits particular properties. Another cool example is think layers of diamond that are being grown as a dome, which has applications in audio.

It is the production and advancement of synthetic diamond (mainly CVD synthesis) that allows for many of these new scientific applications. Technology and uses are currently being explored that were never thought possible before.

Phew... that's quite a run down for why diamond is considered a supermaterial (and why diamonds are so awesome). I haven't even touched on the optical properties that make diamond a beautiful gemstone. Let's leave that for another day...

Want to learn all about laboratory-grown diamonds?

Check out: Ultimate guide to laboratory-grown diamonds