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Big Think asks the tough questions again:

    The 6 strongest materials on Earth are harder than diamonds  
For millennia, diamonds were the hardest known material, but they only rank at #7 on the current list. Can you guess which material is #1?

Thing is, while we can use words like "strong," "tough," and "hard" almost interchangeably in casual conversation, those words have specific meanings in materials science. And there they are in the headline, conflating "strong" and "hard." (I'm the one who muddied the waters with "tough.")

I'll just pause here for a moment while you get juvenile strong/hard jokes out of your system.

Ready?

Ok, good.

Worse, later in the article (spoiler alert), there's even more confusion. So I thought I'd put this up front.

While I do have some background in this from engineering studies, I'm by no means an expert. Wiki has a section   listing materials properties. In particular, "hardness" is scratch resistance, "toughness" is fracture resistance, and "strength" is... well, it's complicated because there are different kinds of strength in materials (compressive, tensile, shear, etc.), and I'm not exactly sure what the article means by strength.

As examples, glass is strong in many ways, but it's not very tough (except for certain specialty glass). Concrete is tough, not very hard (you can scratch it with many metals, as my sidewalk can attest after a snow-shoveling session), and has high compressive strength but low tensile strength. As for diamonds? Hard, but not tough; they'd make a lousy structural material. Their strength is fairly high, but there are many higher-strength materials, which is one reason I felt this article to be somewhat misleading.

One final bit of pedantry before jumping in to the article: As I'm sure everyone is aware, diamond is carbon. Graphite is also carbon, and it's one of the least hard materials known (which is why we can write with it; it needs to be mixed with some other material in pencils so as not to wear down too fast). What we'll see here is that carbon is even more versatile than those two materials would suggest, and that's not even counting its ubiquity in biology.

Although diamonds are commonly known as “the hardest material in the world,” there are actually six materials that are harder.

And here, I'm not sure if the author means simply scratch resistance or not.

For example, hardness, scratch resistance, durability (as many very hard materials are also brittle), and the ability to withstand extreme environmental stresses (such as pressures or temperatures) all factor into how impressive a material can be.

And here we have some muddle again. Durability is a measure of wear resistance; there are brittle materials that are durable but not very hard, such as glass. If glass weren't durable, we wouldn't use it for windows. (To make matters even more muddy, there are types of glass that aren't very brittle.)

On the biological side, spider silk is notorious as the toughest material produced by a plant, animal, or fungus.

Sigh. Spider silk is strong (comparatively speaking), not tough.

While other materials may rank higher on the Mohs hardness scale than diamonds, they’re all easier to scratch than diamonds are. (And, consequently, can be scratched or otherwise damaged through contact with a diamond.)

And this bit makes no sense to me whatsoever. Either I'm missing something, or it's just plain wrong. The Mohs hardness scale, I've known from a very early age, refers to scratch resistance. Like I said, I'm not an expert, so don't just take my word for it.

Now, I've already banged on long enough, but hopefully you get the idea: don't confuse hardness, toughness, strength, elasticity, etc. The article goes on to list the materials that surpass diamond in some way, but I'm still unclear on which material property they really mean.

But I'm sure you'd at least like to know #1 on the list, because superlatives are always interesting:

The #1 hardest material: Graphene

At last, it’s the hardest material of all: a hexagonal carbon lattice that is only a single atom thick.

And so we come full circle: hard diamond, soft graphite, ultrahard graphene. Carbon: is there anything it can't do? But the fun fact, left out of the article as far as I can tell, is that naturally occurring graphite consists of multiple layers of tiny bits of graphene. Sure, the interesting graphene is human-made, but it's not like we didn't know about its existence.

Still, it's a little unclear whether graphene is hard, tough, strong, or some combination of the three. Whatever it is, though, it's damn interesting.