Earliest known appearance in print, attributed to Jefferson: See above.2
Other attributions: None known.
Status: This exact quotation has not been found in any of the writings of Thomas Jefferson. Many reference works claim that he is the source of Delaware's nickname, "The Diamond State," but others simply state that it is called such because of its small size but great importance. The nickname "Diamond State" was current as far back as the 1850s, and thus probably dates to even earlier.3
1. Delaware Federal Writers' Project, Delaware: A Guide to the First State (New York: Viking, 1948), 3.
2. To establish the earliest appearance of this phrase in print, the following sources were searched, as appropriate, for various combinations of the words/phrases "thomas jefferson," "jewel among the states," "delaware," and "diamond state": Google Books, Google Scholar, Amazon.com, America's Historical Newspapers, American Broadsides and Ephemera Series I, Early American Imprints Series I and II, Eighteenth Century Collections Online, 19th Century U.S. Newspapers, American Periodicals Series Online, JSTOR.