Tip-enhanced Raman spectroscopy with a spatial resolution of 25 nm is conducted on a sample of graphite that has been exposed to a 50 keV focused Ga ion beam with a diameter of 20 nm. The G mode located at 1579 cm(-1) does not exhibit a measurable tip enhancement, while the D mode at 1364 cm(-1) is significantly enhanced. A method is proposed to calculate the enhancement factor of anisotropic materials due to the electromagnetic field using the measured signal enhancement induced by the tip.