INCLUSION
An inclusion is a particle of foreign matter contained within a mineral. Inclusions can be solid, liquid, or gaseous. Many inclusions decrease the value of a stone, but some, like rutile forming asterisms in star sapphires and needles in rutilated quartz and tourmalinated quartz, are prized.

INDICOLITE
Indicolite is a green to blue-green variety of tourmaline.

INLAY
An inlay is a piece of material (often stone or glass) that is partially embedded in another material (usually metal) such that the two materials make a level surface.

INTAGLIO
Intaglio is a method of decoration in which a design is cut into the surface. Signet rings are frequently decorated with intaglio, as are seals.

INTERGROWN
Intergrown crystals occur when two mineral crystals grow together and become one.

INVESTMENT COMPOUND
An investment compound is a refractory material (it can withstand extreme heat) which is slightly porous (so that gases from molten metal can escape) and can be formed into a mold (which will be used in metal casting). An example of an investment compound is plaster of paris mixed with silica, boric acid, and graphite.

IOLITE
Iolite, also known an water sapphire, is a transparent, violet-blue, light blue, or yellow-gray mineral. Iolite is pleochroic; a single stone will show many colors (in the case of Iolite, violet-blue, light blue, and yellow-gray). Iolite is not rare and has a hardness of 7 - 7.5. Iolite is found in Sri Lanka, India, Madagascar and Burma.

IRIDESCENT
An iridescent object displays many lustrous, changing colors. Iridescence is caused by the reflection of light from the jewel.

IRIDIUM
Iridium is a metal related to platinum. Iridium and platinum are frequently alloyed together, since the iridium increases the workability of the platinum. Iridium is also used for the points of gold-nibbed pens.

IRRADIATED DIAMONDS
Irradiated diamonds are diamonds that have been exposed to radiation. This changes the diamond's color (as the radiation changes the crystalline structure of the diamond). The change in the diamond is permanent. Older radiation treatments involving exposing the stone to radium; newer treatments bombard the stone with atomic particles in a cyclotron (which accelerates protons, neutrons, or alpha-partices to high speeds). The irradiated stones take on a greenish or an aquamarine hue. Irradiations of diamonds was first done in 1904 by Sir William Crookes, who exposed diamonds to radium, giving them a permanent greenish color; his diamonds are still slightly radioactive (at the level of radium-painted watch). Newer irradiation techniques bombard the crystal with atomic particles in a cyclotron, and then the stone is heated to about 800 degrees Centigrade, producing a stone with very little radioactivity and a permanent color change.