Agarwood Essential Oil
Agarwood is the dark, resinous heartwood of certain species of Aquilaria trees. Agarwood trees are large evergreens that grow in various parts of Asia. Agarwood is a perfume and incense ingredient which has been in use since very ancient times - and its essential oil is described as having a complex, unique and very pleasant aroma.  Genuine agarwood is somewhat challenging to obtain and is regarded as the world's most expensive incense. It has a long history of use in spiritual contexts in Asia. It has also been considered to be an aphrodisiac scent. Its essential oil is typically extracted by steam distillation and used in perfumery.
Several species of aquilaria can produce the agarwood, in addition to Gyrinops trees - however the principal species used is Aquilaria agallocha, also known as Aquilaria malaccensis. 
Agarwood has numerous other names - including Oudh (also spelled Ud, Ude, Aude, Aud, Oodh, Oud, Oude), Gaharu, Agar, Lignum aloes, Aloeswood, Eaglewood, Jinko, Calambac, Agallochum, Aglia, Akyau (Burma), Nwahmi (Siam). You may see the essential oil described as "Attar of Oud".
The cause of the formation of the dark agarwood has been considered something of a mystery.  However according to  , the dark, resinous agarwood heartwood is not formed directly by the tree, but is created in response to an ascomycetous mold. Typically, the Aquilaria tree's wood is pale colored, however when attacked by the mold, the tree produces a dark resin as a form of defense. It is this dark, heavily resinous wood, typically forming in the deep 'core' of the trunk and branches, that is the fragrant agarwood or oud used as a fragrance.
Modern marketing materials seen on the internet cheerfully state that "agarwood has been used as an aphrodisiac for thousands of years". However, agarwood was not widely known in the West until the 18th-19th centuries. Aquilaria is mentioned by Linne in Species Plantarum in 1799, and by Roxburgh as having been donated to the East India Company's Calcutta Botanical Garden in 1805. 
George Watt (sir.) and Edgar Thurston's 1885 "A dictionary of the Economic Products of India" furnishes us with a very detailed and complete description of the agarwood, with many details of its use; however though several medicinal uses are mentioned, there is no mention here either of any aphrodisiacal quality of the resin.  This is typical; and other works, though listing other substances as aphrodisiac, make no mention of this quality for agarwood.
The trusty "Penny Cyclopaedia of the Society for the Diffusion of Useful Knowledge", Vol. 9 of 1837 gives us a lengthy report - including much of value in tracing the history of this tree; and this section, though lengthy, is worth quoting:
"EAGLE-WOOD, one of those substances of which the name, from similarity of sound in a foreign language, has been
converted into another having no reference to its original signification. It is a highly fragrant wood, much esteemed by Asiatics for
burning as incense, and known in Europe by its present designation ever since the Portuguese visited and imported the substance direct from
the Malayan islands and the kingdom of Siam, where it has always been abundant, and long established as an article of commerce. The Malayan
name is agila, whence the wood was called pao-d'agila by the Portuguese, and has since been converted into pao-d'aguila, and
pao-d'aquila, bois-d'aigle, eagle-wood, and agel-hout.
From the Malayan agila has probably been derived the Sanscrit agara, whence we have the Hindu aggur, if not from the more familiar appellation of garoo, by which eaglewood is also known in the Malayan Archipelago. In Persian works on Materia Medica in use in India, we learn from Dr. Royle (lllusir. of Hi mat. Bot., &c.) that several kinds of fragrant wood are described under the Arabic name aod (hand and ud of Garcias), and that he himself obtained three kinds in the bazaars of India, called aod-i-hindee, aod-i-chinee, and and-i-kimaree (evidently the al-cemericum of Arabian authors), and that with the above Hindu a Greek synonyme, agallochee, is also given, and more especially applied to aod-i-kimaree, which is also called aod-i-bukhoor, incense-wood. As agallochee is no doubt a corruption of the agallochum of Dioscorides, described by him as a fragrant wood from India and Arabia, it is interesting to find that the translators from the Greek into the Arabic of the school of Bagdad settled these synonyms at a time when they must have been well acquainted, from their profession and position, with the substances to which both the Greek and Arabic names were applied. Serapion and Avicenna describe several kinds of this fragrant wood, and the latter under both agalugen or aghaloojee, and aod, which in the Latin version is translated Xyloaloe, a name that was applied by the later Greek medical writers to agallochum, whence we have lignum aloes, lign-aloe, and aloes-wood, the origin of which it is difficult, if not impossible, to ascertain, unless we suppose it to be a corruption of agila; for the bitter, scentless, spongy-textured stems of the genus aloe could not afford any substitute for this fragrant wood, or be thought to yield it, at least by the Arabs, who were well acquainted with, and accurately describe aloes, and the place, Socotra, where the best kind is found. Though Dioscorides notices only one, which some supposed to be the Tarum of Pliny, several kinds of agallochum are described by Serapion and Avicenna, which, as it is not possible at present to identify, it is unnecessary to notice, and therefore we shall refer only to the three kinds which have been traced to the trees yielding them, by naturalists who have visited the countries where these are indigenous." 
It is thus evident that the fragrant agarwood has been in wide use since very ancient times: However I can find no mention of agarwood's aphrodisiac quality in the printed works of the European Canon (via Google Books). Might it be that the description of agarwood as aphrodisiac is merely a "marketing add-on", as might befit a particularly pleasant aroma? It is in the literature of the Orient that we must turn if we are to discover the origins of the myth.
In the Book of Proverbs of the Old Testament, there is a mention of Agarwood as a possible aphrodisiac perfume used by a seductress: From the description of Chapter 7-
"I have woven my bed with cords, I have covered it with painted tapestry, brought from Egypt.
I have perfumed my bed with myrrh, aloes, and cinnamon.
Come, let us be inebriated with the breasts, and let us enjoy the desired embraces, till the day appear.
For my husband is not at home, he is gone on a very long journey."
According to Bruce K. Waltke, considered one of the foremost scholars on the Old Testament  , the aloes mentioned is not Aloe Vera, but Lign-aloes, the Aquilaria agallocha.  So there it is.
L.P.A Oyen and Nguyen Xuan Dung's 1999 "Plant Resources of South-East Asia 19: Essential-oil Plants" mentions that the agarwood incense is used as an aphrodisiac in China. 
Agarwood is also used in the Ayurveda medicinal system and is described by Ayurvedic sources as aphrodisiac, as well as having other medicinal properties. 
Agarwood - Scientific Research
A number of papers appear on Pubmed investigating Aquilaria species, in particular reporting the isolation of previously undiscovered molecules - however there is nothing in that great body of scientific literature that investigates any aphrodisiac claim for the tree. A few papers substantiate ancient claims of a sedative effect and some investigate other medicinal properties. Below are listed some of the more interesting and relevant scientific papers for anyone wishing to follow up, perhaps to compare the molecular components with other plants for which the aroma has been considered aphrodisiac, to see what common factors are there:
GC-MS analysis of volatile constituents from Chinese eaglewood produced by artificial methods (2010) -
Aquilarin A, a new benzenoid derivative from the fresh stem of Aquilaria sinensis (2010) -
Two new 2-(2-phenylethyl)chromones from Chinese eaglewood (2010) -
A new 2-(2-phenylethyl)chromone from Chinese eaglewood (2009) -
Studies on perilla, agarwood, and cinnamon through a combination of fieldwork and laboratory work (2008) -
Sedative effects of vapor inhalation of agarwood oil and spikenard extract and identification of their active components (2008) -
GC-MS analysis of volatile constituents from five different kinds of Chinese eaglewood (2007) -
Three 2-(2-phenylethyl) chromones and two terpenes from agarwood (2005) -
Four new 2-(2-phenylethyl)chromone derivatives from withered wood of Aquilaria sinensis (2003) -
Effects of agarwood extracts on the central nervous system in mice (1993) -
Studies on the chemical constituents of Aquilaria sinensis (1989) -
Effects of agarwood extracts on the central nervous system in mice (1993) -
Agarwood and the Environment
Sadly the popularity of the agarwood's fragrance has been so great that it has led to depletion of the trees in the wild, and there have been movements to restrict the harvest of agarwood: In 1995, Agarwood was added to CITES appendix 2 - which places strong restrictions on the movement and sale of agarwood and its derivatives. In many countries, the harvesting of agarwood from natural forests is illegal - however as can be imagined for such a valuable commodity, illegal poaching / harvest of the old growth agarwood trees continues. It is said that in certain countries such as Vietnam, only a few of these old growth agarwood trees remain. 
According to other sources however, the intervention and restrictions placed on agarwood are controversial, causing more problems than they have cured. The contrary argument states that in certain locations it is not the agarwood trees that are being destroyed, but their habitat; and that agarwood trees are being planted as a plantation tree, along with rubber and teak, on the land cleared of indigenous forest! This argument states that the trees themselves are thus not endangered; it is the natural habitat that is threatened. Furthermore, it is stated that outside interests are gaining greater control over the agarwood industry, leaving the original "low-tech" cottage industry struggling.  Conditions vary from country to country and the issue is clearly complex; reminiscent of the problems in the world of Sandalwood.
As might also be imagined, adulteration of agarwood also occurs. It is said by  that much of the agarwood / oudh essential oil that finds its way to the USA is actually made from Boyah, the "uninfected" white wood of the trees. Some reports have stated that agarwood essential oil may contain sandalwood or other oils.
Agarwood - Summary
Agarwood is clearly a very agreeable scent, capable of adding to a sensuous or spiritual atmosphere; it contains some novel molecular substances and there may be something worthy of scientific investigation. However with the environmental challenges surrounding it, it may be preferable to utilize other essential oils - or at the least, know the source and methods of harvesting - if this is even possible, which it may not be. 
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