Agarwood, also known as oud, oodh or agar, is a dark resinousheartwood that forms in Aquilaria and Gyrinops trees (large evergreens native to southeast Asia) when they become infected with a type of mould. Prior to infection, the heartwood is relatively light and pale coloured; however, as the infection progresses, the tree produces a dark aromatic resin in response to the attack, which results in a very dense, dark, resin embedded heartwood. The resin embedded wood is commonly called gaharu, jinko, aloeswood, agarwood, or oud (not to be confused with 'Bakhoor') and is valued in many cultures for its distinctive fragrance, and thus is used for incense and perfumes.
The odour of agarwood is complex and pleasing, with few or no similar natural analogues. As a result, agarwood and its essential oil gained great cultural and religious significance in ancient civilizations around the world, being mentioned throughout one of the world's oldest written texts – the Sanskrit Vedas from India.
As early as the third century AD in ancient China, the chronicleNan zhou yi wu zhi (Strange things from the South) written by Wa Zhen of the Eastern Wu Dynasty mentioned agarwood produced in the Rinan commandery, now Central Vietnam, and how people collected it in the mountains.
Agarwood’s use as a medicinal product has been recorded in the Sahih Muslim, which dates back to approximately the eighth century, and in the Ayurvedic medicinal text the Susruta Samhita.
Starting in 1580 after Nguyễn Hoàng took control over the central provinces of modern Vietnam, he encouraged trade with other countries, specifically China and Japan. Agarwood was exported in three varieties: Calambac (kỳ nam in Vietnamese), trầm hương (very similar but slightly harder and slightly more abundant), and agarwood proper. A pound of Calambac bought in Hội An for 15 taels could be sold in Nagasaki for 600 taels. The Nguyễn Lords soon established a Royal Monopoly over the sale of Calambac. This monopoly helped fund the Nguyễn state finances during the early years of the Nguyen rule.
Xuanzang's travelogues and the Harshacharita, written in seventh century AD in Northern India, mentions use of agarwood products such as 'Xasipat' (writing-material) and 'aloe-oil' in ancient Assam (Kamarupa). The tradition of making writing materials from its bark still exists in Assam.
Aquilaria tree showing darker agarwood. Poachers had scraped off the bark to allow the tree to become infected by the ascomycetous mould.
Agarwood is known under many names in different cultures:
In Urdu (Pakistan) and Hindi (India), it is known as agar, which is originally Sanskrit aguru (in Bengali, also aguru).
In Bangla (Bangladesh) agarwood known as "agar gaach (আগর গাছ)" and the agarwood oil as "agar attar (আগর আতর)".
It is known by the same Sanskrit name in Telugu and Kannada as Aguru.
It is known as chénxiāng (沉香) in Chinese, "Cham Heong" in Cantonese, trầm hương in Vietnamese, and jinkō (沈香) in Japanese; all meaning "sinking incense" and alluding to its high density. In Japan, there are several grades of jinkō, the highest of which is known as kyara (伽羅).
Both agarwood and its resin distillate/extracts are known as oud (عود) in Arabic (literally "rod/stick") and used to describe agarwood in nations and areas in Arabic countries. Western perfumers may also use agarwood essential oil under the name "oud" or "oudh".
In Europe it was referred to as Lignum aquila (eagle-wood) or Agilawood, because of the similarity in sound of agila to gaharu.
Another name is Lignum aloes or Aloeswood. This is potentially confusing, since a genus Aloe exists (unrelated), which has medicinal uses.
In Tibetan it is known as ཨ་ག་རུ་ (a-ga-ru). There are several varieties used in Tibetan Medicine: unique eaglewood: ཨར་བ་ཞིག་ (ar-ba-zhig); yellow eaglewood: ཨ་ག་རུ་སེར་པོ་ (a-ga-ru ser-po), white eaglewood: ཨར་སྐྱ་ (ar-skya), and black eaglewood: ཨར་ནག་(ar-nag).
In Sri Lanka Agarwood producing Gyrinops walla tree is known as "Walla Patta" (වල්ල පට්ට)
There are seventeen species in the genus Aquilaria and eight are known to produce agarwood. In theory agarwood can be produced from all members; however, until recently it was primarily produced from A. malaccensis. A. agallocha and A. secundaria are synonyms for A. malaccensis.A. crassna and A. sinensis are the other two members of the genus that are usually harvested.
Formation of agarwood occurs in the trunk and roots of trees that have been infected by a parasitc ascomycetous mould, Phaeoacremonium parasitica, a dematiaceous (dark-walled) fungus. As a response, the tree produces a resin high in volatile organic compounds that aids in suppressing or retarding the fungal growth, a process called tylosis. While the unaffected wood of the tree is relatively light in colour, the resin dramatically increases the mass and density of the affected wood, changing its colour from a pale beige to dark brown or black. In natural forest only about 7% of the trees are infected by the fungus. A common method in artificial forestry is to inoculate all the trees with the fungus. Oud oil can be distilled from agarwood using steam, the total yield of agarwood (Oud) oil for 70 kg of wood will not exceed 20 ml (Harris, 1995).
Modern Day Research & Uses
The First International Scientific Symposium on Agarwood was held at the Faculty of Forestry, Universiti Putra Malaysia (UPM), during 2013 under the auspices of Associate Professor Dr. Rozi Mohamed 
“It revealed to us the very sophisticated lab work being done on gaharu (Agarwood), not just basic research,” said Dr Rozi, who had received her trainings from the Oregon State University in the US in the fields of forest biotechnology and plant pathology. “The scientists found ISSA 2013 to be very useful because of the collaboration being worked by those from China, Japan, Sri Lanka, Bangladesh, India, Vietnam, Australia, Indonesia and Malaysia,” she said. For Malaysia and China in particular, UPM would be sending one of her students to the labs in Beijing for research on gaharu.
Among the established facts on Agarwood revealed at ISSA 2013 were;
that the laxative properties of the leaves of the tree used as a traditional herbal tea are confirmed and the Japanese researchers concerned planned to launch their product soon;
the resin is produced by the tree in response to attacks by fungi or microbes and that even young trees can be induced to produce the Agarwood / gaharu resin through inoculation and infection;
the leaves when induced in the lab, can produce callus which, when burnt, gave a whiff of Agarwood. This shows that the leaves and stems can be induced to produce the resin;
improved methods revealed by Bangladeshi researchers have led to hefty increase in extraction of Agarwood oil when compared to traditional methods;
higher yields are obtained in infected plants when compared with healthy ones;
the fragrance of Agarwood has yet to be artificially produced;
Aquilaria malaccenis or “karas” trees are typical of the genus as they can be planted as an estate plantation in the interiors for a sustainable economic activity of the minorities. Aboriginal people like the orang asli of Malaysia have been practicing sustainable extraction of gaharu in the wild for generations. The orang asli, the main sources of the highly-prized gaharu from the Aquilaria malaccensis in Malaysia, use their parang or machetes to cut certain parts of the tree to extract the gaharu without killing the trees, enabling them to return every two to three years to extract gaharu again.
Dr Rozi also reported that the most expensive gaharu costs around US$10,000 per kilo and is exported to Japan where the aficionados indulged in the practice of “listening” to the gaharu, meaning they would sit around a smoking gaharu chip in an enclosed room and meditate while relishing the twirling incense. 
Aquilaria species that produce agarwood
The following species of Aquilaria produce agarwood:
Aquilaria khasiana, found in Bangladesh and India.
Overharvesting and habitat loss threatens some populations of agarwood-producing species. Concern over the impact of the global demand for agarwood has thus led to the inclusion of the main taxa on CITES Appendix II, which requires that international trade in agarwood is monitored by TRAFFIC and is subject to controls designed to ensure that harvest and exports are not to the detriment of the survival of the species in the wild.
In addition, agarwood plantations have been established in a number of countries, and reintroduced into countries such as Sri Lanka as commercial plantation crops. The success of these plantation depends on the stimulation of agarwood production in the trees. Numerous inoculation techniques have been developed, with varying degrees of success.
^ abcYule, Henry and Burnell, Auther Coke (1903) "Eaglewood" Hobson-Jobson: A Glossary of Colloquial Anglo-Indian words and Phrases, and of Kindred Terms, Etymological, Historical, Geographical and Discursive (2nd edition) John Murray, London, p. 335, OCLC33186146
^Parfionovitch, Yuri; Dorje, Gyurme and Meyer, Fernand (1992) Tibetan medical paintings: illustrations to the Blue beryl treatise of Sangye Gyamtso (1653–1705) (English edition of Tibetan text & paintings) (2 volumes) Serindia, London, ISBN 0-906026-26-1
^Aromatics, an encyclopedia. 2010. Please note: due to the method of assigning names to medicinal botanicals used in Tibet, it must be considered that woods with similar medicinal properties are named as varieties of the same medicine, and not according to anything akin to the nomenclature of Western botany. Tibetan botanical taxonomy is still in the earliest stage: "white aloeswood" actually refers to the non-aromatic portions of the Indian sandalwood tree; "yellow aloeswood" refers to the scented heartwood of Santalum album. Unique aloeswood is the highest grade of Aquilaria agallocha resin, known in English as Agallochum, while "black aloeswood" is the resin infused wood of the same tree; "brown aloeswood" is actually the scented wood of several Dalbergia species from India and Bhutan.
^ abcdNg, L.T., Chang Y.S. and Kadir, A.A. (1997). "A review on agar (gaharu) producing Aquilaria species". Journal of Tropical Forest Products2 (2): 272–285.
^formerly Phialophora parasitica: Crous, P. W.; Gams, Walter; Wingfield, Michael J.; Van Wyk, P. S. (1996). "Phaeoacremonium gen. nov. associated with wilt and decline diseases of woody hosts and human infections". Mycologia88 (5): 786–796. doi:10.2307/3760973. JSTOR3760973.