Agarwood is a resinous dark wood Aquilaria and related trees (large evergreen trees in Southeast Asia) forms when they are infected with a fungal strain of the Ascomycetes. Before infection, the heartwood is relatively lightly stained; as the infection progresses, the tree in response to attack a dark resin embedded in the heartwood. The resin embedded wood is valued in many cultures for its unique fragrance and used for incense and perfumes. The typical aroma of agarwood can be tested directly by holding a flame against a piece of wood. Agarwood is expensive because it is rare. The unique aroma of agarwood is complex and pleasant, with little or no natural analogues. Therefore has agarwood and its essential oil worldwide has great cultural and religious significance. It is mentioned in the oldest written texts - the Sanskrit Vedas of India. The fragrant wood is used for over two thousand years as incense in traditional celebrations sacred by Buddhists, Hindus and Muslims and as an important part of traditional Ayurvedic, Tibetan and oriental medicine and perfumes. In the third century BC in China mentions the chronicle Nan zhou yi wu zhi (strange things from the South), written by Wa Zhen of the Eastern Wu Dynasty that agarwood is produced in the prefecture Rinan (now in central Vietnam) and how it gathered in the mountains. From 1580, when Nguyễn Hoàng ruler was the central provinces of modern Vietnam, he encouraged trade with other countries, especially China and Japan. Agarwood was exported in three varieties: Calambac (Ky Nam in Vietnamese), Tram Huong (a little harder and less rare) and actually agarwood.