Is it necessary to put "a" in front of the "diamond"? Can we just say "Diamond is the hardest natural substance"?

No it isn't needed, you can (and should) just say "Diamond is the hardest natural substance" there (and similar in part V)

It actually flows better and makes more sense without the "a" in front of it at least in my opinion 

No, it’s not. I’d prefer it without the article. But then you’d probably have to change “it” in the second sentence to “a diamond” or similar. Without the article, you’re referring to the material in the abstract collective sense. With the article, you have a small “piece” of diamond you can work with. So this comes largely down to style and flow, and the writer/editor chose this avenue. Both are fine.

you don't need the "a", and to me it actually sounds weird to put it there

It changes the meaning slightly. A diamond refers to a single stone that you could hold in your hand.💎 “Diamond” is innumerable, like “water” or “gold,” and would refer to the material in general. In this case, they have the same meaning, but it’s important to note nonetheless.

If you say "a diamond," I don't think it is unclear necessarily, but it could also mean that there is one singular diamond somewhere that is the hardest natural substance on earth, but diamonds in general aren't.

A diamond is stating ONE Diamond is stating mutipul

As a native US English speaker, I would not use "A" in the first sentence, but I would change "it" to "a diamond" in the 2nd sentence. (I) is a categorical statement about the substance diamond. It would be silly to claim a specific diamond is the hardest substance unless you tested a lot of diamonds. But (II) refers to a specific diamond--you cannot put the "category of diamond" into an oven.

"A diamond" is an object. "Diamond" is a substance. Saying "A diamond is the hardest natural substance on earth" is nonsensical because "a diamond" is not a substance.

It's actually more correct to omit the 'a'. Here, the hardness is a general property of diamond, the material, not a specific diamond. The way the sentence reads as it is written is like it is stating one *particular* diamond is the hardest substance, but that's wrong because all diamonds have the same hardness.

“A Diamond is..” “Diamonds are..” and “Diamond is..” would all work

Diamond is a substance. A diamond is a gem made of carbon crystal. Using "a" is incorrect here.

Honestly, I understand why it might be weird, in my opinion. I'd think saying, “Diamonds are the hardest natural substance on earth.” makes more sense.

The reason that “a diamond” is wrong is because substances are by nature uncountable. Wood, water, diamond, glass, etc. “are substances”. “A diamond” is not a substance, it’s *made of* a substance. The word “diamond” is notable since we can speak both of “diamond” the (uncountable) substance, and “a diamond” meaning a gem *made of* the substance diamond. We can sometimes do this for other substances, and say “a [substance]” to mean some particular item made of that substance. But it doesn’t mean *any*thing made of that substance; just some particular kind of thing that we all agree on. For example, “a diamond” means only “a diamond gem”; a knife made of diamond wouldn’t really be called “a diamond”. Likewise, “a wood” doesn’t mean a thing made of wood; it means (uncommonly) a forest (which is technically made of wood), and I’ve only heard “a water” used to mean “a bottle of water” (e.g. “Pass me a water from the fridge, please”). “A glass” only means “a drinking glass”. But, “a glass” *used* to mean “a mirror” back in the days when a mirror was called “a looking glass”(or maybe just in the UK, I’m not sure). It also used to mean “an hourglass” when those were popular glass items. So, you can see how *which* thing is meant by “a [substance]” can depend on context and time. For many common substances there’s one representative thing that everyone means by them (“a diamond” means only “a diamond gem”). But for others, the substance might be uncommon enough for “a [substance]” to not mean anything at all. For example, I can’t imagine a layperson talking about “a zinc”—there’s no common zinc item. However, a chemist could easily say “so, imagine if a zinc bonds here…” meaning “a zinc atom”, because the chemist’s specialized context involves a canonical thing made of zinc, namely a zinc atom. (I should note that “a” can just as well be replaced with “the” or any other determiner as needed by the sentence: “the diamond”, “diamonds” to mean multiple diamond gems, etc. I’m just using “a” as an example.) I don’t know what linguistic phenomenon is going on here; I’m just sharing a pattern I’ve noticed as a native speaker. It’s possible there’s a better way to understand this. If someone knows what the rule(s) for this sort of thing in general is, I’d be interested in hearing it. :)

Isn’t “diamond” the material and “a diamond” a faceted gem cut from this material?

Since the very next sentence refers to a specific diamond, in this case it's okay to use "a". It's a stylistic choice to make you think of one specific example of a diamond.

“A diamond” or “diamonds are”

It is possible, but I’m guessing the sentences form a paragraph. So it makes sense to say “a diamond” as in the next sentence this diamond is hypothetically cooked in an oven. It might be more accurate to say “a diamond is made of the hardest…”, but all the sentences are written in a very concise way and this follows the pattern.

Your intuition is correct— we use "diamond" to refer to diamond as a substance and "a diamond" to refer to a physical diamond that exists, and the passage is grammatically incorrect and has a weird meaning.  It sounds like the passage is referring to one specific diamond that has these properties, and then introducing another, separate diamond in the next sentence that is 58 times as hard as the next hardest metal on earth.

Without over analyzing, as a native speaker I would write it as, "diamonds are the hardest..." For your example, I would write "Diamond is the hardest..." For the reasons others indicated

Both are fine. Diamond is a substance, diamonds are a substance, and a diamond is a substance. I personally quite like the book's version because it specifically sets up the mental image of a single gemstone, which leads into the next sentence about putting that gemstone into an oven. It just goes to show how flexible language can be, I suppose.

"A diamond" for me is one shinny stone, so in the context of this quote I'd say not only it is not necessary, but simply shouldn't be used.

It is needed - However, you could say - Diamond is one of the hardest natural substances on earth.

A isn't necessary. Btw are you Turkish?

A diamond is made of...

Most people say there should be no a but I find it normal with the a :/

actually, “a” probably shouldn’t be there. *a diamond* isn’t a substance, it’s an object. *diamond* is a substance.

I think this is actually incorrect as written. A diamond is not a substance. Diamond is.

>Is it necessary to put "a" in front of the "diamond"? Mo. >Can we just say "Diamond is the hardest natural substance"? Yes.

It *should* read, "Diamond is..." rather than, "A diamond is..." because we're talking about the substance itself, as the sentence indicates. If we were talking about one inspecific diamond, it would be correct. The following sentences are talking about an inspecific diamond, so not only is the first sentence wrong, but the second is as well. (And Earth should also be capitalized, or should be preceded by an article. The correct two sentences should be: **Diamond is the hardest natural substance on Earth. But if a diamond is placed in an oven and the temperature is raised...**