Colorful rim toning.
David Hall: The 1937-D is the most common of the three Arkansas commemoratives of that year. The average grade is MS64 or MS65 and circulated examples are virtually non-exsistent. MS66 specimens are scarce and MS67s are definitely rare. Most 1937-D Arkansas halves have a somewhat frosty, somewhat satiny luster. Toning exists in varying degrees so eye appeal is definitely an issue.