At 727 degree Celsius, austenite cannot remain as one single solid phase, it must split simultaneously into two separate solid phases, ferrite and cementite. Whenever one parent solid phase transforms into two new phases together at a fixed temperature and fixed composition, metallurgists call that a eutectoid reaction, and the resulting fine, alternating layered microstructure gets its own special name. Under a microscope, these thin alternating bands of soft ferrite and hard cementite give a shiny, layered look similar to mother of pearl, which is exactly where the name pearlite comes from. So the eutectoid mixture formed from decomposing austenite is pearlite.