Wednesday, October 10, 2012 5:48:37 AM
"Due to it being President Lincoln's hat there is no doubt it was made of the finest beaver short hair. When the pelt was processed, they used some type of a combination comb & cutter to remove all the longer more coarse hair from the beaver pelt. When finished with the complete more elaborate process all that remained was the short, very fine, and softer under hair." - From the Smithsonian website.
The ribbon is indeed silk. But apparently there is some discrepancy between the NMAH description and the Smithsonian one.
The Abraham Lincoln Library and Museum states that "There`s also a private vault housing one of the three existing beaver hats Lincoln wore".