Which of the following rocks is the result of a chemical process?

The correct answer, quartzite, is a rock that forms through a chemical process known as metamorphism. Specifically, it originates from sandstone, which is primarily composed of quartz grains. When sandstone undergoes intense heat and pressure, typically during tectonic processes, the silica in the quartz grains recrystallizes. This metamorphic process solidifies the rock into quartzite, making it extremely hard and durable.

In contrast, granite is an igneous rock that forms from the cooling and solidification of molten magma beneath the Earth’s surface. Sandstone is a sedimentary rock made up of sand-sized grains, primarily silica or quartz, that have been cemented together. Coal is also a sedimentary rock, formed from the accumulation and compaction of plant material over millions of years. While all these rocks are created through geological processes, quartzite's formation is specifically tied to chemical and physical changes that occur under high temperature and pressure conditions.