The one you are referring to is imitation.
Since you own genuine pearls I will give you a simple test you can do. Real pearls are cooler than imitation pearls made from coated glass or plastic beads.
Assuming all the pearls have been in the same room for a while (same ambient temperature), compare the temperature of the strand you are in doubt about by touching it briefly to your lips, then repeating the test with a real strand. The real strand will feel cooler on your lips.
Lips are very sensitive to temperature, which is why we suggest touching them to your lips.
If you own a jeweler's loupe (10x magnifier) and use it to examine the surface of the pearls, you will see the real pearls have a smoother surface texture than imitation pearls. In this photo of Renee Newman's, the imitation pearl is on top.
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