Diamagnetism is a basic property of matter. It is always presented. The diamagnetism is due to the interaction of the magnetic field with the motion of the electrons in their orbits. It is called "Larmor diamagnetism" (N.W.Ashcroft, N.D. Mermin, Solid State Physics, Saunders College Publishing, USA, (1976)), and is temperature independent.
A rough estimation of the diamagnetic susceptibility is Xdia = 0.5* M * 10-6 emu/mol, where M is a molecular weight of the compound.
A short theory with tables. Ref
Pierce W. Selwood: Magnetochemistry, Interscience Publishers, New York, 1956, p 78.
Pascal Tables from O. Kahn, Molecular Magnetism, VCH Publishing, 1993