沉香 Agilawood

沉香指的是沉香屬植物流出的樹脂與木質結合在一起的混合物。有這些異化物的木質部份稱為「沉香木」、而皮質部份的稱為「沉香皮」。因為沉香的密度較高，它能完全沉入水底或可半浮半沉。 

沉香木（Aquilaria agallocha, or agilawood）指的是沉香屬裡的樹木，但是沉香木這個俗稱也普遍形容一些產生了「結香」效果之後而有了「沉香」味的特定樹木^[1]。

沉香木植物樹心部位當受到外傷或真菌感染刺激後，會大量分泌帶有濃郁香味的樹脂，古代稱為瓊脂。這些部分因為密度很大，又被稱為「水沉香」。

在世界上很多地方，沉香木是珍貴的香料，中國俗語有謂「一兩沉香一兩金」之說，被用作燃燒薰香、提取香料、加入酒中，或直接雕刻成裝飾品。

嚴格地來講，「沉香」與「沉香木」是兩種不同的概念，類似「牛」和「牛奶」: 是牛不一定會有牛奶。沉香是沉香木樹幹被真菌侵入寄生，發生變化，經多年沉積形成的香脂。人們熟悉的檀香等帶香味的木材有所不同，沈香不是一個樹木的品種。而是一種特殊的樹木（瑞香科沈香屬的樹）在遭受需擊、風折、蟲害、動物的破壞後，為了保護樹體而分泌了樹脂，樹脂又聚集於傷口周圍。此時，樹木的傷口會被真菌侵入寄生，在菌體內物質酶的作用下，樹木薄壁細胞儲存的澱粉產生壹系列變化，形成香脂，最後經過長時間的累加沈積，產生香脂與木材纖維的混合固態凝聚物，這才是真正的沉香。 

 Formation of agar wood occurs in the trunk and roots of trees that have been penetrated by an wood and oily resin feeding bug. The insect belongs to the family of the Ambrosia beetle named Dinoplatypus Chevrolati (Gen. Sc. Prof. Stephan-Alexander Peter, Lembah Sari National Park Mt. Rinjani Lombok, Indonesia). A life long infection may 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 colour, 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 specie are infected an produce agar/aloes wood. A common method in artificial forestry is to inoculate all the trees with the fungus. It produces a "damage sap" and is scientifically referred to as "fake" aloes/agar wood.^[23] Oud oil can be distilled from real (minimal 45+ years of naturally (Dinoplatypus Chevrolati) fermenting due to its continuously infection)aloes/agar wood status using steam, the total yield of agar wood (Oud) oil for 70 kg of wood will not exceed 20 ml (Harris, 1995).