Bone piece coming thru gum post wisdom extraction and dark spot around tooth root.dentist says irritation, not infection of tooth.could that be right?

Bone yes. Spot decay: Common for bony spicules to pop up following extraction and are a source of irritation than infection. Dr could pluck it out. Dark spot is on next tooth? If so it could be decay and would need a filling. See your dentist for brown spot on root.

Bone fragment: This is probably a small bone fragment working its way out from the extraction site causing a little gum irritation. If it doesn't exfoliate within a few days, go back to the dentist that performed the extraction for a follow up exam.

Yes.. causes:: It is not uncommon for there to be a small spicule of bone or even a small tooth fragment following the extraction of a tooth. Most of the time this separates and no further treatment is necessary. As an alternative, the dentist can remove it and hasten the healing process. It might also be bone not covered by gum. If not gone within a week, have it re-evaluated by your dentist.

Yes: small bone spicules are not that uncommon and generally resolve themselves.

Sequestra: It's not uncommon that after having a wisdom tooth extracted a patient will notice one or more small hard fragments of the bone. See your dentist for assistance, because it is your dentist's goal to help you throughout your extraction's healing process. Good luck.