China's control over crucial rare earth materials has been a "threat for a long time" to Western supply chains and the U.S. should create a strategic reserve of the metals, University of Pennsylvania professor emeritus of finance Jeremy Siegel told CNBC's " Squawk Box " Monday. "It's scandalous that we don't have a rare earth strategic reserve [and] that we let China monopolize 90% of refining rare earth materials," Siegel said. "Where were we, realizing the importance of these?" Siegel's