Short answers, pulled from the story.
In geology, a rock, also called a stone, is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way it formed. Rocks make up the Earth's crust and most of its interior.
The three main groups of rock are igneous, sedimentary, and metamorphic. Igneous rocks form when magma or lava cools and solidifies. Sedimentary rocks form by diagenesis and lithification of sediments, and metamorphic rocks form when existing rocks are transformed by high pressure and temperature without significant melting.
About 65% of the Earth's crust by volume consists of igneous rocks. Of these, 66% are basalt and gabbro, 16% are granite, and 17% are granodiorite and diorite. Sedimentary rocks make up about 7.9% and metamorphic rocks 27.4% of the crust by volume.
Silica content is the most important chemical criterion for classifying igneous rock, with alkali metal oxide content next in importance. Silicate minerals account for about 95% of the Earth's crust and about one-third of all known mineral species.
Metamorphic rock forms at temperatures greater than 150 to 200 degrees Celsius and pressures greater than 1500 bars, always higher than those at the Earth's surface. Such conditions occur, for example, when continental plates collide. The original rock, called the protolith, recrystallizes without significant melting.
Humans and other hominids have used rock for at least 2.5 million years, making lithic technology one of the oldest continuously used technologies. Soft sedimentary rock was quarried for construction as early as 4000 BCE in Egypt, and stone built fortifications in Inner Mongolia as early as 2800 BCE.
Anthropic rock is synthetic or restructured rock formed by human activity, such as concrete, which has been developed since Ancient Rome. Geologist James R. Underwood has proposed anthropic rock as a fourth class of rocks alongside igneous, sedimentary, and metamorphic.