I was reading about the Arkansas diamonds just the other day.

http://en.wikipedia.org/wiki/Crater_of_Diamonds

It's the only diamond site in the world open to the public. Although most diamonds come from kimberlite pipes, those in Crater of Diamonds comes from lamproite magma which originates in the upper mantle.

BTW, the 3.09 carat Strawn-Wagner diamond (later cut to 1.09), found in 1990, is the only diamond in the world to be graded perfect flawless by both the AGS and GIA.

http://en.wikipedia.org/wiki/Shirley_Strawn

The Uncle Sam Diamond is the largest discovered in the United States at 40.23 carats.

http://en.wikipedia.org/wiki/Uncle_Sam_%28diamond%29

Brian