Fluorine is a gaseous element which in the form of fluorides is fairly widely distributed in nature.
It occurs as a trace in most ground-water, usually less than one part in a million. If a bone or tooth lies for thousands of years in a moist gravel or sandy formation, it gradually absorbs wandering fluorine ions from the ground-water. Once they enter the bone substance they are not released, unless the whole bone becomes dissolved. The process goes on continuously, and the fluorine-content of the bone or tooth increases in course of time. This fact provides rather a neat means of distinguishing fossilized bones of different ages occurring at a particular place.