Saint Ambrose [4th century CE] (Hexameron, Book 5, chapter 11.33): Water also produces a 'golden fleece' and the sea coasts are the source of a wool which is similar in appearance to the metal just mentioned. Its color has not been duplicated up to now by those who apply to woolen goods different types of dye. For that reason human ingenuity is unable to compete with the natural products of the sea. We are aware of the care and attention given to the less costly sheep's wool. No matter how perfect it is, under no circumstances do we find wool that comes naturally dyed. Here is a color that is natural a color never yet approached by the application of dyes. And to think that this [golden] fleece is a fish! Moreover, the shell-fish that yields the purple which distinguishes a king is itself a marine animal. - [Savage translation, 1961]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.8): The golden fleece (aureum vellus) is also nourished by the sea, as Ambrose testifies, and the shores produce wool in the kind of metal mentioned. The color of which none of those who colored the skins with different dyes, no matter how skilled, could yet imitate. This kind of wool is believed to have been that wool, for which infinite thousands of men burned cities and depopulated the country in the region of the Trojans, as told in the most ancient of the Greeks tales. - [Badke translation/paraphrase]