An alkaline solution obtained with potassium oxide is easily altered and ceramic objects will often take on a very characteristic iridescence if they come into contact with the earth or other chemical agents.
(This explains the iridescence seen in the peacock plate! I wonder if this iridescence was ever deliberately engineered by burying items or soaking them in special solutions)
Cobolt was imported from Europe via Venice. Muzarrad is a black stone used to make black, probable utilising antimony and comes from the mountains of Jajarm in Khorasan, Eastern Iran.
Turquoise is obtained from copper oxide and an alkaline glaze (well ozygenized). Copper oxide and a lead glaze (in reduction) produces green hues as well.
Bowl, 12th century, Iran. Met Museum of Art, Accession number: 29.160.12. Showing a slight hint of the iridescence.