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).
Where diamonds are found in the United States - Wyoming. Diamond Kimberlite pipes have been found in Wyoming, near the Colorado-Wyoming state line area referred to as the Aultman, Ferris and Schaffer kimberlites. The Herkimer Diamond Mine and the Ace Of Diamonds Mine - New York.
Diamonds are usually level 16 and bedrock, and is near lava a lot. I have found that in abandoned mine shafts there's some by lava especially. And iron is usually everywhere, even above the ground. So that means it doesn't have to be in caves or holes.