Since as early as the 4th century AD tea has been compressed into various shapes. Traditionally, they are called black tea in China. The dense shapes were more efficient to transport than loose leaves and the compacted leaves were more resistant to spoilage on the way to the border regions of the Chinese empire. They were used as forms of currency in trade with the nomadic tribe of Mongolia and Tibet and are still used today in those areas. Most cakes, bricks, bowls, cones, melons or other shapes are made of Pu Erh tea, a unique, earthy tea that is preserved at a higher moisture level. Aged for 20, 30 years or even more, some are sought after buy collectors. Authentic Pu Erhs come from Yunnan Province. The black tea bricks originate from Hubei Province. Hubei bricks were well know, especially by the northern tribes.