Black tea is a variety of tea that is more oxidized than the oolong, green, and white varieties. All four varieties are made from leaves of Camellia sinensis. Black tea is generally stronger in flavor and contains more caffeine than the less oxidized teas. Two principal varieties of the species are used, the small-leaved Chinese variety plant , also used for green and white teas, and the large-leaved Assamese plant , which was traditionally only used for black tea, although in recent years some green has been produced.
While green tea usually loses its flavor within a year, black tea retains its flavour for several years. For this reason, it has long been an article of trade, and compressed bricks of black tea even served as a form of de facto currency in Mongolia, Tibet, and Siberia into the 19th century. Although green tea has recently seen a revival due to its purported health benefits, black tea still accounts for over ninety percent of all tea sold in the West.
