All pearls are, by definition, calcareous concretions because their natural nacre is calcium-based. However, some "pearls," which are defined as nacreous calcareous concretions, are not technically pearls. Two prime examples are pearls from the conch and the melo melo snail. These gems are considered a pearl within the trade, but not by definition as they are not made by nacreous coatings. Their creation is almost identical to how true pearls form, with the exception that the calcareous concretion that makes the gem is not true nacre.