There's no single hardest rock on Earth. The strongest types, on the other hand, include quartz in their makeup: quartzite, quartz sandstones, rhyolites and granites, according to the MadSci Network.
While many cite diamonds as the hardest rock on Earth, this is incorrect. Diamonds are not a rock. They are a mineral. Minerals are a "naturally occurring, inorganic (not alive) solid," as MadSci explains. They also have a crystal-shaped structure.
Rocks are a little different. They contain one or more minerals, but are not minerals themselves. Granite, for instance, is made of quartz and other minerals, often feldspar and biotite.