Thermoplastics, in basic terms, are melt-processable plastics (materials that are processed with heat). When enough heat is added to bring the temperature of the plastic above its melt point, the plastic liquefies (softens enough to be processed).
In the U.S., diamonds have been found in Arkansas, Colorado, Wyoming, and Montana. In 2004, the discovery of a microscopic diamond in the U.S. led to the January 2008 bulk-sampling of kimberlite pipes in a remote part of Montana.
The oldest and most famous diamond mine in North America is the Crater of Diamonds which is just south of Murfreesboro, Arkansas. Diamonds were first suspected in the area and in 1889, state geologist John Branner gave the area a thorough surface search and didn't find anything.