Glossary of Jewelry Terms: O

Old-European Cut: A term applied to the earliest form of a circular, girdled round stone or brilliant. It is characterized by a very small table, a heavy crown, and usually great overall depth. Improperly referred to as an "old-mine cut".

Old-mine Cut: (a) An early form of brilliant cut with a nearly square girdle outline. (b) Incorrectly applied to a somewhat more modern style of brilliant cut that also has a much higher crown and smaller table than the modern brilliant cut, but whose girdle outline is circular or approximately circular—a style of cutting that is more properly called a “lumpy stone” or and old-European cut.

Onyx: A semi precious stone that is black or white in color lends itself to flat jewelry creation such as cameos, since it has a layered structure. Onyx belongs to the "chalcedony" family of minerals, which are somewhat porous stones. It has a hardness of between 6.5- 7 on the Mohs Scale.

Opal: Opals, known for their iridescent, luminous qualities, are adored by many. Opals contain a large amount of water and need to be cared for properly since experts warn of potential cracking. This semi precious stone contains a wide-ranging mixture of colors that produce a fire-like quality, which are actually inclusions which can refract hues in a rainbow of colors. For more information about the history of opals, visit Fabulous Facets Gem History (use your browser's "back" key to return here)
Opaque (pronounced "oh payk"): Not transparent or translucent. An opaque stone will not allow any light to pass through it. 

Open Back Setting: Setting in which the back of the stone can be seen; if a rhinestone, usually the crystal is unfoiled.

Opera Length: A necklace which is 48 to 90 inches long.

Ore: A metal bearing mineral from which metal can be profitably mined or extracted.

Oriental Pearl: A pearl that has formed naturally with no human intervention. See also "Pearl".

Ormolu: A term referring to gilded bronze or brass mounts. From the French for "ground gold"

Oval Cut: Faceted, elongated stone, round at both ends.

Oxidation: A chemical process in which a metal, such as silver, is blackened or tarnishes, as a reaction to sulphur and oxygen.

Oxide: A compound containing one oxygen atom per molecule.

Oxidize: The act of combining with oxygen molecules to make an oxide.Oxidized metal is rusted.

Oxygen: A nonmetallic element that is normally a colorless, odorless, tasteless gas that constitutes 28 percent of the Earth's atmosphere. Oxygen combines with many other elements easily. These compounds are called oxides and make up about half the solid matter on Earth, making oxygen the most abundant element present in the Earth's crust.