Black tea is a type of tea that is more oxidized than oolong, green and white teas. All four types are made from leaves of the shrub Camellia sinensis. Black tea is generally stronger in flavor than the less oxidized teas. 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.