Koboku (fragrant wood) (香木)
Koboku is kind of wood that gives off a pleasant aroma. Jinsui Koboku (also known as Jinko or Chinko) and Byakudan are famous. In Japanese, "Ko" means aroma.
The thinly split wood, when heated, gives off an enjoyable aroma. Even when it isn't heated, Byakudan gives off a pleasant smell; therefore, it's commonly used for sculptures of Buddha, folding fans and Juzu (strings of rosary beads).
History
Koboku dates back to May (April in the old lunar calendar) 595, when Jinsui koboku had washed ashore on Awaji-shima Island; this is the oldest historical reference to the koboku in Japanese culture. It is noted in a Japanese document that the koboku that washed ashore was thrown into a fire and a sweet aroma began to arise. The wood was presented to the court, and thereafter became an item of appreciation. Among the treasures at Todai-ji Temple Shoso-in Treasure Repository, is a large piece of koboku called 'Ojukuko,' measuring 156 cm long and 43 cm wide at its widest and weighing 11.6 kg. This is thought to have come into Japan before the Kamakura period, and thereafter men of power tried to obtain pieces of it. The sections taken by the eighth shogun Yoshimasa ASHIKAGA, of the Muromachi bakufu (Japanese feudal government headed by a shogun), as well as Nobunaga ODA and Emperor Meiji, are shown with signage. According to Todai-ji Temple's record, Nobunaga cut off two pieces off the wood, each about 3 cm around.
In April 1992, Zenkoku Takimono Senko Kumiai Kyogikai (National Council of Fragrant Woods and Incense Association) established April 18 as Incense Day, based on the notation in the Nihonshoki (Chronicle of Japan) that Jinsui koboku first washed ashore in Japan in April (in the old lunar calendar) and disassembling the kanji '香' into 一十八日 (eighteenth).