It isn't too hard t imagine what could be a 'collectible' cultured pearls - one that is desirable and guaranteed to remains rare. Something close to the biological limits of the species (to insure that new technologies don't make the current 'rarity' run of the mill with the next, improved harvest!), devoid of defects and with at least one quality characteristic over the top... There is a thread about Aussie and NZ pearls with a bunch of examples: some large, some perfect, some surprisingly colored, some all of the above ... walnut sized baroque pearls with crazy orient. Abalone mabes perfect and big as a coin.
That sort of thing.
I have not been too interested but this question so far, so I am missing completely a rather important piece of information: where on Earth can one
resell such collectibles after all! To be 'collectible' something must retain some liquidity and value (at least), methinks. Looking at auctions, the favored cultured pearl items are necklaces and even those do not sell every time. Natural pearls do. I am not implying or assuming anything - just don't know.
The nicest pearl I would consider 'collectable' that I have seen in person (and must have influenced the way I am thinking of the matter) was a huge, golden baroque with the shape vaguely reminding a rounded cloud set in a necklace with a prominent signature on it. The peal had pleasing variations of the color from pinkish cream to deep golden, great lustre, quite a bit of iridescence of various shades, and some surface texture; not a 'perfect' pearl, but one anyone could love. No conspicuous blemishes (although some may have been covered by the setting) and remarkable size - perhaps ~ 3cm wide and not much smaller in Evey other direction.
I cannot remember the exact price - high, but not shocking. The peal was perfectly suited for the design, appealing and hard to replace though.
Just 2c.