Final answer:
Diamond is the hardest material among quartz, feldspar, topaz, and diamond itself. It is composed of carbon atoms forming strong covalent bonds, resulting in a very hard crystalline structure.
Step-by-step explanation:
Diamond is the hardest material out of the following options: quartz, feldspar, topaz, and diamond itself. It is composed solely of the element carbon and has a very hard crystalline structure. Each carbon atom forms four single bonds with other atoms, creating a three-dimensional network that gives diamond its exceptional hardness.