Flint Ridge contains deposits of Vanport flint in rocks from the Pennsylvanian Period, 299-320 million years ago. These deposits are the largest and purest occurrence of flint in the state, and at this site the flint occurs in layers 10 to 12 feet thick. Vanport flint is notable for its wide array of colors. Although it ranges from white to black and is usually light gray to milky white, often mottled with patches and streaks, it also appears in bright red, yellow, green, and blue, giving rise to the name Ohio's "rainbow" flint. Because of its unusual beauty and historical importance, Flint Ridge flint was named Ohio's official gemstone in 1965.