Black tea is a variety of tea that is more processed and more oxidized than the green, oolong and white tea varieties. After picking, the leaves are withered and then bruised by rolling, inducing oxidation and creating their characteristic black color and complex flavor. The exact level of oxidation depends on the specific intentions of the tea master responsible for producing the tea. The leaves are dried or “fired” by being heated in an oven, thus reducing the moisture content. Black tea is generally stronger in flavor and contains more caffeine than the less oxidized teas. While green tea usually loses its flavor within a year, black tea retains its full-bodied flavor for several years. Compressed bricks of black tea once served as a form of de facto currency in Mongolia, Tibet and Siberia all the way into the 19th century. India, Sri Lanka and China are the most famous producers of high quality black tea.