Occurrence
It is the area where a particular mineral is found.

Ochre
It is an amorphous, yellow to red substance that is composed of iron compounds, like hematite and limonite.

Octacube
It is a name given to a three dimensional polyhedron which is a combination of a cube and octahedron.

Octahedron
It is a term given to the eight sided polyhedron. All the sides of it are equidimensional and bisect at the same angle. Minerals of such shape belong to the isometric system. It is also known as Octahedra or Octahedrals in plural forms.

Oil of Vitriol
It is the synonym of sulfuric acid.

Oily (luster)
It is the synonym of greasy luster.

Oligocene
It is a period of geological time which is from 28.5 to 23.8 million years ago.

Opalescence
It is the effect which is seen in few minerals, like opal, causing it to exhibit a glimmer of different colors if rotated or seen in different angles.

Opaque
One which is not able to transmit light, hence disabling it to let an object be seen through it.

Optical properties
It is the physical properties of a mineral or gem having to do with optics, like dispersion, absorption spectra, regfractive index, asterims, and dichroism.

Ordovician
It is a period of geological time which extends from 495 to 440 million years ago.

Ore
It is for those material that has a valuable constituent, commonly a precious metal, giving it a value and making it a profitable for extraction. Example: Calaverite, extracted for the gold it contains.