Fluoride is an ionic compound containing fluorine, which is the single most reactive element; it is naturally found in many rocks.  

About 90 percent of the fluoride added to public water supplies comes from silicofluorides, chemicals produced mainly as byproducts from the manufacture of phosphate fertilizers, according to the CDC.

Fluoride is added to public water supplies at an average concentration of about 1 part per million (1 ppm), or slightly below. Naturally occurring fluoride concentrations in surface waters depend on location but are generally low and usually do not exceed 0.3 ppm. Groundwater can contain much higher levels, however.

http://www.fluoridealert.org quick facts: