The means of making porcelain was successfully kept a secret by China for about two centuries after it became hugely popular in the west, where it was only duplicated in the first decade of the 18th century. The popularity of this material even led (at least in English) to it becoming synonymous with the name of the country which was its only source; hence the term "fine china" for example.

Very controlled methods of import and export. The silk trade was a huge deal and merchants were often searched. It took hundreds of years for someone to finally be able to get ahold of and smuggle out silk worms and mulberry seeds.

Would merino wool be an example of this? I know the breed was developed by the 1400s, and exporting a live merino sheep from Spain was a capital crime until the 1700s.

Would it have been common knowledge that the wonderful wool that came from Spain was from a particular breed of sheep that could not legally be exported? Or was the wool's origin a mystery to people in the surrounding countries?

Greek Fire comes to mind. There's a nice write up on it here: https://www.reddit.com/r/AskHistorians/comments/1mfit4/how_was_the_recipe_for_greek_fire_lost/?

Alum, used in the textile industry, comes to mind. In medieval and early renaissance times it was only available from one place in Italy and the pope controlled it, and held a monopoly. One of the main benefits when King Henry VIII broke with papacy was his ability to break the monopoly.