Is an ultrasound much more likely to detect a gallstone than an ordinary xray? also a gallstone can sometimes cause pain but can it ever create a serious medical problem if there is no treatment?

Yes: X-RAYs don't reveal most gall stones but ultrasound does. Untreated gallstones can cause acute inflammation of the gall bladder or pancreas,and jaundice as well as perforation of the gall bladder with dangerous peritonitis. Many gall stones cause no problems,however but there is no guarantee that they won't.

Yes: Gallstones are usually cholesterol and not calcified therefore no visible by x-Ray. Gallstones can get in common bile duct and cause obstruction and also can cause rarely cause intestinal obstruction

Gall stones: Ultrasound is more sensitive test than plain xray for the diagnosis of gallstones. It can cause pain ans serious problem of obstruction of the bile duct and pancreatitis, Sometimes it may be sitting there without any symptoms.