Bauxite was designated the official state rock of Arkansas in 1967. Arkansas has the largest bauxite deposits in the United States (located in Saline County). Bauxite contains aluminum ore, used to make soft drink cans, aluminum foil, and many other products. The photo below shows a bauxite boulder near Little Rock, Arkansas.
The plaque says:

BAUXITE BOULDER

This bauxite boulder of high quality aluminum-bearing ore was placed March, 1943, to symbolize Arkansas's contribution to World War II. The boulder weighs nearly twenty tons. It was brought from the Dulin Bauxite Mine near Sweet Home, Pulaski County. The state of Arkansas is furnishing over 98 per cent of all bauxite mined in the United States for aluminum production. This tablet is made of aluminum manufactured from this type of ore. Homer M. Adkins, Governor CG. Hall, Secretary of State

Bauxite rock with high-quality aluminum ore in Little Rock, Arkansas
(text on the plaque is above picture) -
photo © kat selvocki / lemony kickit (shiny red type) on Flickr - noncommercial use permitted with attribution / no derivative works