The FTIR spectrum of PURE graphite (without defects) should be completely flat because there are only polyaromatic layers. Any defect (such as oxygenated groups) will induce a loss in symetry of aromatic planes, causing some absorption in IR spectrum.
There is some IR spectrum of diamond in a paper of mine (Diamond and Related Materials, 2006).
If the diamond was treated under UHV it contains some unsturared conjugated C=C bonds (absorbing around 1900-2000 cm-1). It is treated under H2, it is H-terminated (C-H stretching and bending at 2800-3000 and around 1400 cm-1 are visible).