I'm very confused here. According to rule K2, "If a part category does not list the part you wish to use, then it is not allowed." Now this
should include the Exotic Materials list too... However, this leaves the question:
Is there anything else not explicitly included in the list that is considered "exotic", and if so, where is the boundary? 
Then I refer to the Part Use Flowchart, where it states indirectly that, if an item is not listed under the Exotic Materials list, and it is not otherwise prohibited, then it is allowed.
It seems to me that this section is handled differently from the rest of the restricted materials list, and that essentially, any material not explicitly listed as exotic is allowed, or else we are to assume that only those materials listed are considered exotic.