Areca nut stains

Betel nuts are 'palm nuts' from the areca tree. These hard nuts are chewed casually like chewing gum by islanders and is a permanent feature of the cultures of the Pacific. Betel nuts are chewed and harvested by millions of people from India, Vietnam, Sri Lanka, Indonesia, Philippines, Marianas, American Samoa, Beleau, Bangladesh. The trees are found growing in moist ground and produce clusters of green fleshy nuts which mature into yellow and then brown hard nuts. 

The Betel nut also works as a psychoactive drug. People chew it for stress reduction, feelings of well-being and heightened awareness. Betel nut is chewed regularly by at least 10% of the world population, imported by immigrants users wherever they settle, and is the fourth most widely used addictive substance. Users often chew it all day long, causing all sorts of side effects such as red-stained teeth, damaged gums and can increase the chances of acquiring cancer of the mouth, pharynx, esophagus and stomach.

Info from here and here