So with all the above answers in mind, I would like to say that I don’t consider diamond to be a rock. Like Jharp’s mineral composition opinion, and from my mineralogy class, I think that the definition of a rock is that it has experienced lithification and cementation. Diamond is just an altered mineral, with the same chemical composition as graphite and stishovite.
To give a final answer for your question, and keep in mind that I did graduate in the Geology department, Quartzite is the hardest rock. It is different from Quartz in that it is a metamorphosed sandstone (and sandstones are hard). This means that it had to go through revisions by being bakes and hardened more after it cooled. Although I would agree that any rock containing Quartz does make it harder, in the case of Granite. Comparing mineral composition diagrams, Quartzite would rate higher than Granite. It is stronger than Quartz alone because metamorphosis transformed the crystallography of Quartzite to be tighter and more compact, even if it is made up of almost all Quartz.
I can’t actually believe it’s the hardest rock. But you learn something new everyday!
Diamond is a mineral, pure carbon (and not the same composition as stishovite, which is silicon dioxide). Rocks are aggregates of minerals, so diamond(s) can be parts of rocks. Quartz, with its hardness of 7, is by no means “among the hardest rocks” and besides, quartz too is a mineral. You can have monomineralic rocks – much sandstone is just quartz. Quartzite is just quartz, and would have the hardness of quartz.
A rock that is a very hard natural abrasive is emery, composed almost entirely of corundum and magnetite. Emery was used by early Egyptians to carve granite.
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