Agarwood, aloeswood or gharuwood is a fragrant dark resinous wood used in incense, perfume, and small carvings. It is formed in the heartwood of aquilaria trees when they become infected with a type of mould (Phialophora parasitica). Prior to infection, the heartwood is odourless, relatively light and pale coloured; however, as the infection progresses, the tree produces a dark aromatic resin, called aloes or agar (as well as gaharu, jinko, oud, or oodh; not to be confused with bukhoor), in response to the attack, which results in a very dense, dark, resin embedded heartwood. The resin embedded wood is valued in many cultures for its distinctive fragrance, and thus is used for incense and perfumes. The aromatic qualities of agarwood are influenced by the species, geographic location, its branch, trunk and root origin, length of time since infection, and methods of harvesting and processing 
One of the main reasons for the relative rarity and high cost of agarwood is the depletion of the wild resource. Since 1995, Aquilaria malaccensis, the primary source, has been listed in Appendix II (potentially threatened species) by the Convention on International Trade in Endangered Species of Wild Fauna and Flora. In 2004, all Aquilaria species were listed in Appendix II; however, a number of countries have outstanding reservations regarding that listing.
First-grade agarwood is one of the most expensive natural raw materials in the world, with 2010 prices for superior pure material as high as US$100,000/kg, although in practice adulteration of the wood and oil is common, allowing for prices as low as US$100/kg. A whole range of qualities and products are on the market, varying in quality with geographical location, botanical species, the age of the specific tree, cultural deposition and the section of the tree where the piece of agarwood stems from. Oud oil is distilled from agarwood, and fetches high prices depending on the oil's purity. The current global market for agarwood is estimated to be in the range of US$6 – 8 billion and is growing rapidly.
The odour of agarwood is complex and pleasing, with few or no similar natural analogues. In the perfume state, the scent is mainly distinguished by a combination of "oriental-woody" and "very soft fruity-floral" notes. The incense smoke is also characterized by a "sweet-balsamic" note and "shades of vanilla and musk" and amber (not to be confused with ambergris). As a result, agarwood and its essential oil gained great cultural and religious significance in ancient civilizations around the world, being described as a fragrant product as early as 1400 BCE in one of the world's oldest written texts – the Sanskrit Vedas from India. Agarwood is highly revered in the seminal texts of Hinduism, Christianity, Buddhism, and Islam .
Dioscorides in his book Materia Medica (65 CE) described several medical qualities of agarwood (Áγαλλοχου) andmentioned its use as an incense. Even though Dioscorides describes agarwood as having an astringent and bitter taste, it was used to freshen the breath when chewed or as a decoction held in the mouth. He also writes that a root extract was used to treat stomach complaints and dysentery as well as pains of the lungs and liver. Agarwood’s use as a medicinal product was also recorded in the Sahih Muslim, which dates back to approximately the eighth century, and in the Ayurvedic medicinal text the Susruta Samhita.
As early as the third century CE in ancient China, the chronicle Nan 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.
During the sixth century CE in Japan, in the recordings of the Nihon Shoki (The Chronicles of Japan) the second oldest book of classical Japanese history, mention is made of a large piece of fragrant wood identified as agarwood. The source for this piece of wood is claimed to be from Pursat, Cambodia (based on the smell of the wood). The famous piece of wood still remains in Japan today and is showcased less than 10 times per century at the Nara National Museum.
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.
Agarwood is known under many names in different cultures:
- In Cambodia, it is called "chann crassna". The fragrance from this wood is called "khloem chann" (ខ្លឹមចាន់) or "khloem chann crassna". "khloem" is hard wood, "chann crassna" is the tree species Aquilaria crassna in khmer language.
- In Hindi it is known as agar, which is derived originally from the Sanskrit aguru .
- In Bengali, agarwood is known as "agor/agoro gach (আগর গাছ)" and the agarwood oil as "agor/agoro attor (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 "deep scent" and alluding to its intense scent. 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 Arab countries. Western perfumers also often 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 Assamese it is called as "sasi" or "sashi".
- The Indonesian and Malay name is "gaharu".
- In Hong Kong it is often called Aloes wood 
- In Papua New Guinea it is called "ghara" or eagle wood.
- In Thai it is known as "Mai Krishna" (ไม้กฤษณา).
- In Tamil it is called "akil" (அகில்) though what was referred in ancient Tamil literature could well be Excoecaria agallocha.
- In Laos it is known as "Mai Ketsana" (ໄມ້ເກດສະໜາ).
- In Myanmar (Burma) it is known as "Thit Mhwae".
- In Sri Lanka Agarwood producing Gyrinops walla tree is known as "Walla Patta" (වල්ල පට්ට)
There are seventeen species in the genus Aquilaria, large evergreens native to southeast Asia, and nine are known to produce agar wood. 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. The gyrinops tree can also produce agarwood.
Formation of agar wood occurs in the trunk and roots of trees that have been penetrated by an insect feeding on wood and oily resin, the Ambrosia beetle (Dinoplatypus chevrolati by</ref> Gen.Sc.Prof. Stephan-Alexander Ebu Can Peter, 2017). A mold infection may then occur, and in response, the tree produces a salutary self-defense material to conceal damages or infections. While the unaffected wood of the tree is relatively light in color, the resin dramatically increases the mass and density of the affected wood, changing its color from a pale beige to yellow, orange, red, dark brown or black. In natural forests, only about 7 out of 100 Aquilaria trees of same species are infected and produce aloes/agar wood. A common method in artificial forestry is to inoculate trees with the fungus. It produces a "damage sap" and is referred to as "fake" aloes/agar wood. Gen.Sc.Prof. Stephan-Alexander Ebu Can Peter, 2017
The composition of agarwood oil is exceedingly complex with more than 150 chemical compounds identified. At least 70 of these are terpenoids which come in the form of sesquiterpenes and chromones; no monoterpenes have been detected at all. Other common classes of compounds include agarofurans, cadinanes, eudesmanes, valencanes and eremophilanes, guaianes, prezizanes, vetispiranes, simple volatile aromatic compounds as well as a range of miscellaneous compounds. The exact balance of these materials will vary depending on the age and species of tree as well as the exact details of the oil extraction process.
Aquilaria species that produce agarwood
The following species of Aquilaria produce agarwood:
Conservation of agarwood-producing species
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 be monitored. Monitoring is conducted by London-based TRAFFIC (a joint WWF and IUCN programme). CITES also provides that international trade in agarwood be 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 Malaysia and Sri Lanka as commercial plantation crops. The success of these plantations depends on the stimulation of agarwood production in the trees. Numerous inoculation techniques have been developed, with varying degrees of success.
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- 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.
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- The genus Gyrinops, is closely related to Aquilaria and in the past all species were considered to belong to Aquilaria. Blanchette, Robert A. (2006) "Cultivated Agarwood – Training programs and Research in Papua New Guinea", Forest Pathology and Wood Microbiology Research Laboratory, Department of Plant Pathology, University of Minnesota
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Snelder, Denyse J.; Lasco, Rodel D. (29 September 2008). Smallholder Tree Growing for Rural Development and Environmental Services: Lessons from Asia. シュプリンガー・ジャパン株式会社. p. 248 ff. ISBN 978-1-4020-8260-3. Retrieved 8 October 2010.
Jung, Dr. Dinah (1 January 2013). The cultural biography of agarwood (PDF). University of Heidelberg: HeiDOK: Journal article: JRAS. pp. 103–125. Retrieved 30 October 2016.
|Wikimedia Commons has media related to Agarwood.|
- Hong Kong herbarium factsheet of Aquilaria sinensis
- Etymology of agarwood and aloe
- "Sustainable Agarwood Production in Aquilaria Trees" at the University of Minnesota
- Traditional and Medicinal Uses of Aquilaria / Agarwood https://web.archive.org/web/20130908181916/http://www.cropwatch.org/agarmed.htm