Missouri designated the nut of the Eastern black walnut tree (juglans nigra) as the official state nut in 1990. The black walnut (also called American walnut) is native to eastern North America, growing mostly alongside rivers. The black walnut is a deciduous tree with grey-black, deeply furrowed bark and is considered very high quality wood.

Black walnuts are used in ice cream, baked goods and candies. The nut shell provides a soft, gritty abrasive used in metal cleaning and polishing and oil well drilling, and is also used in paint products and as a filler in dynamite.

Extracting the kernel from the fruit of the Black Walnut is difficult. The nut shell is covered by a thick husk that contains a dark, staining, strong-smelling juice. The juice will often be a yellow brown at first, but quickly becomes a deep black-green color when exposed to the air. The shell often protrudes into the meat, which means whole kernels cannot usually be obtained. But the flavor of black walnuts is superb and worth the effort.

Source:

Missouri Symbols - The State Nut: Missouri Kids
State Symbols of Missouri - State Tree Nut: Missouri SOS
Juglans nigra - Black Walnut: Wikipedia.org

Links:

American Food and Agricultural Symbols
American Food Holidays