Fluoride is of interest because of its effects on dental enamel and bone. It comes from the element fluorine, the 17th most abundant element in earth's crust. It is a gas and never occurs in free form in the nature. Fluorine exists only in combination with other elements as fluoride compound, which are constituents of minerals in the rocks and soil. As water passes over rock formations it dissolves fluoride compounds that are present, creating fluoride ions. As a result of this, small amounts of soluble fluoride ions are present in all water sources including oceans. Fluoride is present to some extent in all foods and beverages, but concentrations vary widely.