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.
Diamonds are formed as an allotrope of the element carbon, at depths greater than 90 miles. The pressure and heat at this depth was critical to the formation of the diamonds that are mined at the surface today. At crustal pressures, the stable form of carbon is the allotrope graphite.