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Occurrence

The area where a particular mineral is found.

Ochre

Amorphous, yellow to red substance composed of iron compounds such as Hematite and Limonite.

Octacube

Three dimensional polyhedron that is a combination of a cube and octahedron.

Octahedral

Shaped as an octahedron.

Octahedral cleavage

Type of cleavage exhibited on minerals of the isometric system that crystallize as octahedrons. The method of cleavage is that flat, triangular "wedges" peel off of the existing octahedron. Example = Fluorite.

Octahedron

Eight sided polyhedron; all sides are equidimensional and bisect at at the same angle. Minerals shaped as octahedrons belong to the isometric system. Plural = Octahedra or Octahedrals

Oolite

Any mineral that is Oolitic.

Oolitic

Aggregate composed of very small, spherical particles.

Opalesce

The present term describing something displaying opalescence.

Opalescence

Effect seen in a few minerals, chiefly opal (hence its name) which cause it to exhibit a glimmer of different colors when rotated or seen in different angles.

Opalization

The common habit of Opal that it replaces material, such as wood, shells, and other minerals.

Opalize

To be chemically altered to Opal. See Opalization

Opaque

Not able to transmit light, which in effect disables it from letting an object be seen through it. (See Transparency in mineral properties for more information.)

Optical

Having to do with optics.

Optical properties

Physical properties of a mineral or gem that have to do with optics, such as dispersion, absorption spectra, refractive index, asterism, and dichroism, just to name a few.

Optics

Branch of physics that deals with light and the electromagnetic radiation; certain areas are dispersion, absorption spectra, reflection, and refraction.

Ore

Material which has a valuable constituent, usually a precious metal, that gives it value and makes it profitable for extraction. A fine example is Calaverite, which is extracted because of the gold it contains. "Ore" also refers to the precious material that was extracted from the ore.

Organic

Composed of carbon compounds; being from the source of living organisms.

Orthorhombic (crystal system)

Any mineral that falls under the following specifications belongs to the orthorhombic crystal system: Three axes, all are unequal in length. All three axes are at 90° to each other. (See Crystals in mineral properties for more information)

Outcrop

Bedrock revealed at the surface of the earth.

Oxidation

The process of undergoing a chemical change through exposure to oxygen. See also oxidize.

Oxidation minerals

Minerals that form after being altered from being exposed in the oxidation zone.

Oxidation zone

Area of a deposit where the rock is exposed to air and therefore is affected by wind, rain, pressure, and air, which chemically affects the minerals embedded in the rock and alters them to secondary minerals.

Oxides

Group of minerals that are compounds of one or more metallic elements combined with oxygen, water, or hydroxyl (OH). The oxide group contains the greatest variations of physical properties. Some are hard, some soft. Some have a metallic luster, others are clear and transparent. The Oxide group is divided into the Simple Oxides Hydroxides Multiple Oxides

Oxidize

Property exhibited in certain minerals that cause them to tarnish, or discolor upon contact with air. Also term used to describe the chemical alteration of one mineral into another mineral through oxidation.

Oxidized

Having gone through a chemical change as a result of exposure to oxygen.