Nacreous pearls are the pearls that we mostly talk about and see. These are the South Sea Pearls, Tahitians, and Freshwater Pearls. Non nacreous pearls are grown in conch shells. There are many varieties, but the main ones are the Conch Pearls and the Melo Melo Pearls. 

 You know, by now, that the nacreous pearl's luster is caused by its optical properties and chemical composition. Their brilliance is caused by the reflection of light off the nacre's surface.

Conch pearls react to light in different ways. Conch pearls have no nacre. Instead they have a porcellaneous surface. They have fibrous prismatic crystals that are perpendicular to the surface of the pearl. This creates the "flame" like vision. Think of moire silk. That's the kind of visual effect you see on these pearls.

Nacreous and non nacreous pearls have the same chemical composition. It's just a difference of their polycrystalline structure.

You might think that these pearls would be numerous because you have seen many conch shells, but it takes about 10,000 conch to produce one pearl and then again many more to produce an exceptional conch pearl.

If you ever get a chance to see one, do it. I do not have one...yet, but I have seen some magnificent jewelry in the Christie's auctions that I was able to work a long, long time ago, last year. Hemmerle, Tiffany, and Boucheron have worked with them in their jewelry, as well.