Agarwood or oodh is a dark resin

Resin

Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

ous heartwood that forms in Aquilaria

Aquilaria

Aquilaria is a genus of fifteen species of trees in the Thymelaeaceae, native to southeast Asia. They occur particularly in the rain forests of Indonesia, Thailand, Cambodia, Laos, Vietnam, Malaysia, Northern India, the Philippines, Borneo and New Guinea. The trees grow to 6-20 m tall...

and Gyrinops trees (large evergreens native to southeast Asia

Southeast Asia

Southeast Asia, South-East Asia, South East Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia. The region lies on the intersection of geological plates, with heavy seismic...

) when they become infected with a type of mold. 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.