Conch Pearls
A Conch Pearl is a non-nacreous, calcareous concretion produced primarily by the Queen Conch (Strombus gigas). Conch pearls often exhibit a flame-like pattern due to concentrically arranged calcium carbonate plateles in a lamellar fasion.

Conch Pearls, The Pearl That’s Not Really A Pearl, in the Technical Sense
Conch pearls (pronounced "konk") contain no nacre, so technically they are not actually pearls at all. Instead, the gems are calcareous concretions, similar to kidney stones in humans. They are produced by the Queen conch mollusk (Strombus gigas), which lives primarily in the Caribbean near southern Florida.

Conch Pearls Are A Very Rare Occurrence
No one has yet devised a method for culturing conch pearls, so every one you see will be a natural pearl. Therefore, they are extremely rare and valuable. It is estimated that only one in 10,000 conchs produces a pearl, and that less than 10 percent of those are of gem quality.