For months i've had a grainy white bump on my lower gum. the dentist referred me to an oral surgeon for mri, etc.... trying to avoid $$$ if it is common and not harmful. how common is this?

Not all that common: But maybe it is something that could be biopsies instead of MRI . You don't know till you go. Obviously it is not normal and you need together it taken care of.

MRI ?: Something seems off. I personally can't see why an MRI seems warranted from what you describe as clinical symptoms alone. Consider a second opinion or restate why the OS thinks it's necessary.

Very common : Such a grainy white bump may be caused by a gum or tooth abscess, tumor, bone exostosis, irritation,or just a normal anatomical structure. Proper evaluation and diagnosis is the key. See your oral surgeon and follow his/her instructions.