ICON : A religious amulet worn on the body, usually bearing an image of a saint or a god.

IGNEOUS ROCKS : Rocks formed from erupted volcanic lava or solidified magma.

IMITATION GEMSTONE : Material that has the outward appearance of the gem it is intended to imitate, but which has different physical properties. 

INCLUSIONS : Markings or foreign bodies found within a stone.

INTAGLIO : Design in which the subject is cut lower than the background.

INTERGROWN : When two or more minerals grow together and become interlocked.

IRIDESCENCE : Reflection of light off internal features in a gem, giving rise to rainbow-like play of colours.