Ruby is a type of rock known as a metamorphic rock. More specifically, it is a variety of the mineral corundum, which is mainly composed of aluminum oxide. The rich, red color of ruby comes from traces of chromium present in its composition.
Rubies are formed under high pressure and temperature conditions deep within the Earth, often occurring in a metamorphic setting where limestone and other sedimentary rocks have undergone transformation. Because of this unique formation process, rubies are classified as gemstones and are highly valued in the jewelry industry.