What would cause a spot to show up on my jaw in a dental xray. It is about the color of the teeth on the xray and a near perfect circle.?

Hard to say. It would be so hard to tell without seeing the actual image, but if it is a perfect circle and appears whiter than the teeth, it would most likely be something man-made, or something that got in the way of the image such as an ear-ring in a pan image. This is a great question to ask your dentist in person so you can look at the image together. If he or she does not know, a specialist can help.
X-ray diagnosis. What comes to mind first is the metal button that is used on some digital x-rays that get scanned when the film is inadvertently reversed. Other possibilities are an artifact, some external jewelry, salivary stone, or one of many types of odontogenic growths. A trained dentist should be able to easily answer that question for you. We can only guess not being afforded the ability to see the image.
Density. The denser an object, the whiter it appears on an x-ray. Could be one of a number of benign anatomic variations or could be that the x-rays were blocked by something metallic, such as an earring. Ask your dentist to review the x-ray with you. He/she will probably know exactly what it is. Can't tell by just description over the comp;utter.
Several things. Teeth and bone appear as different shades of white on a x-ray. The spot that appears almost the same color as your tooth could be an extra tooth, a cementoma or hypercementois, which are aberrant accumulations of a tooth component. Other options could be dense bone as in a torus, or from a chronic infection, a calcification in the soft tissue, or a foreign body.
Cementoma. This could be mistakenly diagnosed as dental periapical pathology. They about the color of the teeth and pose a challenge for the dentist in their diagnosis and management. See OMS for diagnosis.
Possible earring. Possible piercing--ie-earring in nose, ball in tongue. Prior to taking x-rays, the patient is asked to remove all piercings so that they do not obscure the image needed of the dentition. If this is not the case, check with the dentist to make sure no foreign matter is lodged.
Several possibilitie. Depending on the location if its the same radiopacity that a tooth and located at tha root tip it can be a cementoma, a benign lesion. If its a bit clearer it can be an exostoses or excess bone. If this appearance is causing some problems a biopsy and surgical removal might be necessary.
Abscess. It could be an abscess, depending on the location. Looking at the area and palpating the area should provide a clear answer. See a dentist with a good amount of experience as over the years we tend to see it all.