Chang Shih-nan 張世南. A native of 鄱陽 P'o-yang in Kiangsi. Flourished under the Sung dynasty, about A.D. 1230. Author of Youhuan jiwen 游宦紀聞 "Memories of my Travel in Search of an Office". His book includes 108 short chapters that discuss a vast amount of themes, ranging from stories of history to customs and habits, stories of personal affection, antiques and collectibles, literature, lexicography and etymology, archaeology, calendar, mathematics, medicine etc.

Taken from:  Paul Wheatley Geographical notes on some commodities involved in Sung maritime trade.

A Liang (3) of the highest quality (Ambergris) never fetching less than 100000 strings of cash (1) on the Kuang-chou (2) market while even the second quality was worth 50-60000 a Liang. (3)

Ambergris was included among the fine quality goods, and its purchase and resale defined as a government monopoly.

(Given as is quoted in the; Kin-ting ku-kin tcu-shu tsih-clfing published in 1723 )
The Yiu hwan ki wen' instructs us that the most precious of all perfumes is dragon's spittle, and that the inhabitants of Ta-shih land used to watch the vapors arising for half a year or even two or three years from the same spot of the sea. When they vanished, this was a token that the dragons which had been sleeping there all the time had gone away. Then the people went to the spot in order to gather the saliva of those dragons. Where they never failed to find five, seven, or even ten liang (3) of ambergris.

According to another explanation, found in the same passage, the dragons lived in whirlpools in the open sea. The spittle which they emitted was hardened by the sun, and these hard pieces were blown ashore by the wind. When fresh it was white, gradually it became purple, and finally black.

A third explanation, namely that 'the spittle of the dragon is eaten by a certain fish the excrement of which is washed up on the seashore as ambergris'

Another version: The shores of the great eastern sea of the Arabs were the habitat of a plant whose flowers resembled those of the hibiscus {mu-ju jung)  If a certain large fish having fed on dragon's spittle (4) changed subsequently to devour these flowers they would  induce vomiting, and the ejected spittle would float on the surface of the sea, where it would harden to form ambergris.

Note: What is being talked about is Ambergris; these explanations of where Ambergris comes from are all taken from Islamic sources. A proof that the Chinese never collected Ambergris on their own shores but imported them through Islamic merchants. The Arabs collected Ambergris on all Indian Ocean coasts (including East-Africa); but the importance of early texts pointing to Ambergris in China is also that we have mentions of  black people arriving in China at the same time as the Ambergris; the Arabs were also importing black slaves, (who might also have come from South East Asia and / or Africa)

(1) strings of cash: used as a superunit of the Chinese cash (amounts fluctuated a lot)

(2) Kuang-chou: Guangzhou formerly Canton; A city of southern China on the Pearl River Delta near the South China Sea.

(3) Liang; traditional Chinese weight unit, about 37.8 grams, the same as a tael; now 50 grams

(4) dragon's spittle: Ambergris