What is the collective noun of pearls?

That's an interesting one. The most common and widely accepted collective noun is a 'string of pearls'. This is what you would hear most often, particularly when they're arranged as jewellery. You could also correctly say 'a rope of pearls', which usually suggests a much longer or more lavish necklace. So, while 'string' is the classic answer, 'rope' is a perfectly good alternative depending on the specific arrangement of the pearls. Both are quite correct.