http://www.blissofwellness.com/product-detail.php?detail=65

Oud or Agarwood or Aloeswood (Aquilaria agollocha, A. malaccensis, A. crassna) is an evergreen tree native to the forests of Northeast India, Bhutan and Southeast Asia; Viet Nam, Kambodja, Philippines, Indonesia, Papua New Guinea. The tree reaches a height of about 40m with a diameter of about 60cm. 

It bears sweetly-scented snow-white flowers. For the oil to exist, the tree needs to be infected with a fungi; Phialophora parasitica. The tree produces this oleoresin to protect itself against the fungi by saturating the heartwood. The longer time a tree is infected, the deeper and darker the oleoresin gets. This can take up to a 100 years and inevitably ends with the tree dying. By this time the heartwood is almost black, deeply saturated and scented by the oleoresin. The heartwood of an uninfected tree is light and pale-colored.