Fluoride

Fluoride is a mineral in your bones and teeth. It’s also found naturally in the following:

water
soil
plants
rocks
air
Fluoride is commonly used in dentistry to strengthen enamel, which is the outer layer of your teeth. Fluoride helps to prevent cavities. It’s also added in small amounts to public water supplies in the United States and in many other countries. This process is called water fluoridation.
ref: Healthline