Tribe woman with bloody lips (caused by chewing betelnut), and blackened teeth, a cultural and traditional sign of beauty (Bảo Lạc, Vietnam)
Betelnut ("cau" in Vietnamese) is is the fruit of the Areca tree. It is wrapped with a "lá trầu" leaf (a different tree) and chewed for its exhilarating lift. Betel quids chewing has been claimed to produce a sense of well-being, euphoria, heightened alertness, sweating, salivation, a hot sensation in the body and increased capacity to work.
Just like alcohol, tobacco, coffee and sakau, betelnut is a culturally-approved psychoactive drug.
See the Betelnut Wikipedia page.
See the "lá trầu" leaves Wikipedia article.
Diu Nghiem wrote: Her blackened teeth was NOT caused by chewing betelnut and betel leaf but by using some special chemical. Blackening tooth chemical is usually natural material which varies from the North to South of Vietnam. The process might take a couple of day in order to get the perfect shiny black.
Men and women did that because of the same reason which caused other people whitening their teeth (they consider black teeth to be a sign of beauty). After the occupancy of France in Vietnam blackened teeth is not popular anymore.
More information about the blackened teeth custom:
"Healthy Black Smiles" by Barbara Cohen