Illusion Setting - a setting in which the surrounding metal's surface is shaped or cut to appear to be part of a diamond. This type of setting is used to enhance the size of small diamonds.
Inclusion - a foreign body or irregularity within a gemstone.
Inlay - placing shaped pieces of material into the surface of an object so that both the material and object are level. A popular type of inlay is different colors of opal set into jewelry.
Intaglio - a design carved into or below the surface an opaque gemstone such as black onyx. This type of carving shows the image in relief. It would be the opposite of a cameo.