Icosahedral
One which is shaped as an icosahedron.

Icosahedron
It is a name given to twenty sided polyhedron where all sides are equidimensional. Minerals such shaped belongs to isometric system. Plural: Icosahedra or Icosahedrons.

Igneous
It is one of the three main groups of rocks on the Earth. They have crystalline textures and appears to have consolidated from a silicate melt (magma).

Igneous rock
It is a type of rock from the volcanic origins. It can be glassy, crystalline or both.

Impure
One which contain impurities.

Impurity
It is an item present but not part of the integral structure of a mineral and also sometimes changes the optical properties of the mineral.

Inclusion
It is a term used for the materials which are locked inside a mineral while it is forming, eg. rutilated quartz.

Index of refraction
It is the synonym of refractive index.

Indiscernible
One which is not noticeable.

Inertinite
It is one of the three macerals which make coal.

Infiltration
It is a pseudomorph which occurs when some of the atoms of a mineral are replaced by different atoms and forms another mineral.

Inorganic
One which do not contain any of the organic substances.

Inosilicates
It is a group of silicate minerals which have their tetrahedrons form single or multiple chains. It has two oxygen atoms of each tetrahedron part of its neighbouring tetrahedron forming long and thin chains.

Insoluble
One that is not soluble.

Intermediary
It describes a mineral which is in midway between two mineral of a series, like Olivine is in midway between Forsterite and Fayalite in the Olivine Group.

Intrusion
It is a body of igneous rock which is emplaced into pre-existing rocks, either along some of the structural feature like a fault or by deformation and rupturing of the invaded rocks. Adjective: Intrusive.

Intrusive rock
It is an igneous rock which has been formed underground from magma being stuffed into the cracks.

Ion
It is an atom which becomes either positively or negatively charged due to the loss or gain of an electron.