I am a lover of pearls, a consumer, not a seller. I collect Mikimoto pearls. In my opinion, pearls do not hold their value. They are worth their most the day you purchase them as they are in the best shape. A pearl, unlike a diamond, will "wear out" in time. Diamonds are timeless and never wear. This is unlike a pearl which is a natural gem, and is only at its best brand new, unworn. I wear my pearls all the time, and I am sure I put dents and scratches in them. That said, I adore them and love wearing them.
Pearls only have value if they have provenance (e.g. Elizabeth Taylor's La Peregrina pearl necklace or Marilyn Monroe's Mikimoto necklace). I am no Elizabeth Taylor nor Marilyn Monroe, so I know my pearls will not resell for much on the open market. I buy pearls for today, not for resale, and because I love pearls. Speaking of old pearls, I saw Elizabeth Taylor's La Peregrina Pearl Necklace in real life as I went to the exhibition. To me, without knowing it's provenance, it is not a pearl I would want. It does not have nacre, but that is expected given that pearl is from the 16th century.
I hope this helps. Buy pearls because you love them, not because they will increase in value.
If you want to invest, don't by pearls. If you want to enjoy pearls and look elegant, buy them now and enjoy them. Don't wait. Pearls are at tiemless look.