Here is what I see with the first strand that makes it look imitation:
1. A lack of French wire (a high quality professional finish) at the clasp.
2. Colors, size and surface are too uniform for real pearls.
3. The knots are large, which is often the case with imitations because many imitation pearls have larger drill holes than real pearls. (This is because real pearls are sold by weight and smaller drill holes preserve weight.) However high end imitations may also have small drill holes; my Majoricas do.
4. I don't see blemishes. Real pearl strands have some blemishes.
Examine the 3-strand necklace closely. Photograph any blemishes you see, close up and in focus.
Examine the drill holes. Imitation pearls sometimes have these qualities near drill holes (although less likely with good fakes. I never see these with my Majoricas).
• swirls of extra coating
• peeling/flaking of the coating
• beveled surface near the drill hole
Examine the clasp. Are there any metal marks indicating the metal?