A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z 
NATURAL
Natural can refer to a gemstone or a pearl. A natural stone is called such because it has not been subjected to any treatments.
A natural pearl is one that has grown through a natural irritant getting into an oyster, as opposed to a cultured pearl which will have been seeded with a bead nucleus.