Ivory, as in made from elephants.
You probably need at least 9 elephants for a decent tower. Not sure how you convince them to stay still though. Probably feed them some Mnokeys.
Note: There are export restrictions on transporting elephant products across national borders.
Also the secluded environment of theoretically-oriented Universities and academic departments. Quantum physics is is performed in the IvoryTower, mechanical engineering is performed in the RealWorld and clinical medicine is performed in MeatSpace.