Very rarely (about 1 in 10,000 conchs), a conch pearl may be found within the mantle. Though these pearls occur in a range of colors corresponding to the colors of the interior of the shell, pink specimens are the most valuable. These pearls are considered semi-precious, and a popular tourist curio. The best specimens have been used to create necklaces and earrings. A conch pearl is a non-nacreous pearl (formerly referred to by some sources as a 'calcareous concretion'); it differs from most pearls that are sold as gemstones in that it is not iridescent. The specific weight of the conch pearl is 2.85, notably heavier than any other type. Due to the sensitive nature of the animal and the location of the pearl-forming portion of the snail within the spiral shell, commercial cultivation of pearls is considered virtually impossible.