Now as many know, The nether is a representation of Hell, which in much folklore is made of "brimstone." Brimstone is in fact a name for something real though, Sulfur, but that doesn't work, as sulfur is yellow, not red. Now when netherrack was first added, it was named Bloodstone, which, just like brimstone is a name for a real world, the mineral Heliotrope, a "cryptocrystalline" quartz rock, with a black green and red colored stone, which would work, with the quartz of the nether and all, but not entirely. It's color messes this up, it's mainly a black-green with spots of red in it, so that doesn't work, but it can if we must. Now if we look at Netherrack, It has a red color, a low hardness, and is heat resistant, with a dull color, but that didn't get me anywhere, looking up red minerals. So I went to look somewhere else, and got the idea to look at it's composition in the education edition of Minecraft. When put in the material reducer, you get 64 silicon, 18 oxygen and 15 mercury, as well as some magical mystery materials, the silicon and oxygen implies that it is a silicate, and silicates are in fact one of the main ingredients in quartz, Now the mercury, what about that? well if I bring back Brimstone, Sulfur, if you mi the two you get mercury sulfide, AKA, Cinnabar, the most common form of mercury in nature. It is associated with Volcanic activity and is a dull brick red, just like netherrack. Therefore I believe that Netherrack is Cinnabar with Silicate in it, forming Quartz. That was more difficult to discover than you might think.

I was gonna do endstone as well, but the netherrack feels like enough, plus netherrack is much more useful to explain other things than endstone is

Also this explains what Nether Quartz ore is, The same as netherrack, just with more silicate that has crystalized.

https://en.wikipedia.org/wiki/Sulfur
https://en.wikipedia.org/wiki/Heliotrope_(mineral)
https://en.wikipedia.org/wiki/Quartz
https://en.wikipedia.org/wiki/Silicate
https://en.wikipedia.org/wiki/Cinnabar