I have a white hardened layer over my gums in my mouth. originally my dentist said it was a callous but it has spread now. is it oral cancer?

Biopsy: It may be time to see an oral surgeon for an evaluation and may be biopsy of the lesion so that the tissue may be examined by a pathologist.

Gum area: There are many things that can cause this kind of thing. An oral surgeon has the training and knowledge to determine what it is. See an oral surgeon

No: Have the calculus (tarter) professionally removed...it causes gum/bone recession. Have your Dentist teach you how to keep it from reforming.

DDS evaluation: Get a definitive diagnosis from your dentist. You mention that the dentist stated that calculus is present, after a through prophy, your issues should be remedied. If the dentist feels that you need to follow through with scaling and root planing or an evaluation with a periodontist- do it.