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I've heard that diamond is the hardest natural known material but, on Google search, I found that it can easily be broken by a hammer as it's not tough.
So, what is difference between hardness and toughness? According to me, both should be the same, and if both are different than what is the toughest known substance?
Hardness and toughness are not the same
Hardness and toughness are very different qualities in materials and are weakly related.
Hardness is strongly related to the more well-defined quantity of stiffness which measures how easily a compound can be deformed under stress. Glass and diamond are very stiff materials, for example. If you try to poke them with something they resist deforming to accommodate your poke. (Hardness is not perfectly aligned to stiffness because of small scale microstructures in many materials, but this is good enough for now).
Toughness is a vaguer term for materials and there isn't a simple way to measure it. This is partially because it varies depending on circumstances in a way that stiffness does not and it is a property of the overall structure and not just the materials that make up the structure. Understanding it requires some insight into why things break (and why other things don't). we need to know a lot about the small scale structure of materials not just the substances involved. For example, stainless steel (used in knives and forks) is a well-known tough material but cast iron is brittle. Both are mostly made of iron. The differences are in the crystalline structures.
One key property of tough structures is that cracks don't propagate. So a stick of glass will break easily as will a glass fibre. But a bundle of glass fibres embedded in an epoxy resin can be very tough (because the cracks in individual glass fibres are not propagated through the epoxy resin). Some tough metals can adjust the micro defects in their crystalline structures to absorb the strain that would otherwise propagate cracks. Some very soft compounds are very tough because the deform so easily that it is hard to start cracks, nylon rope for example.
Diamond is very very stiff. But it has no protection mechanism against cracks. So, like glass, once a crack has started it doesn't take a lot of energy to cause it to spread, so it may be stiff but it isn't tough.
A fuller explanation of this took material scientists and engineers a long time to work out would require a whole book to do it justice. Luckily that book has been written and is called The New Science of Strong Materials. It is well worth a read.
Diamond has cleavage planes. If you want something nearly unbreakable, try nephrite, which is a tough form of jade used by the ancient Aztecs to make axe heads! Actinolite is another related "tough as steel" mineral. These minerals are made up of interlocking strands (actinolite) or microscopic fibers (nephrite). But diamond is a regular geometric lattice, so if you hit it at the right angle, it will break. Still, you can't make a scratch on glass with nephrite -- but diamond will do the job. That's because diamond is "harder" than glass (which is mostly silicon).
If toughness is defined as the ability of object A to damage object B, then there's no such thing as the toughest object, let alone the toughest substance. For example, a steel saw can cut a block of wood in two. The very same block of wood, used as a hammer, can break a steel saw in two.
Even if we consider similar geometries, the type of damage would depend on the kind of action involved. A block of diamond would scratch a block of iron, and diamond vise would slowly flatten an iron block, while a fast impact would rather shatter diamond.
Also, you can easily flatten or break a small piece of steel with a steel hammer.
I haven't read all the answers that have been written, but the ones I have read are very good.
This is going to be a simple one:
Yes indeed, Diamond is the hardest substance known and it refers to the resistance to be scratched. In other words, try using a metal point to scratch it and it won't be possible. While you can use diamond to scratch or indent other substances.
It does not mean that it can't be destroyed with a heavy blow, the resistance to this has another name and diamond does not possess it.
It all is reduced to the type of solid in which it crystallizes, normally a stiff and not flexible crystal lattice becomes brittle, easy to break. But as it is very dense, small distance between atoms and or ions, it does not allow penetration.
