Obsidian: A dark glassy volcanic rock formed by very rapid solidification of lava. In primitive cultures the sharp glassy edges were used for shaving and cutting. It was used for cutting the umbilical cord of new born babies.

Octahedral: A geometric object with eight plane surfaces.

Oiling: The process of applying mineral oil to a stone, such as an emerald, to mask inclusions and enhance the color.

Old European Cut: A round Brilliant cut with 58 facets, similar to the old mine cut, first appearing in around the 1880's. It is characterized by a deep pavilion, high crown, and an open culet.

Old mine cut: See Mine Cut.

Oligoclase: (Also called Plagioclase) A triclinic feldspar made of soda-lime.

Olivelia Shells: A kind of seashell used for Heishi necklaces.

Olivine: See Chrysolite and Peridot.

Omega chain: Unlike most chains which are a series of linked rings, an Omega chain is made up of rectangular smooth rounded metal plates set side by side and crimped along the ends onto a strip of metal mesh. The end result is a fairly stiff, yet flexible, circle which needs to be worn high around the neck since it is not nearly as attractive when viewed from the edges.

Omega back: These work a bit like lever back earrings, in that they have a hinged lever on the back of the earring near the bottom, but in this case the lever ends in an O shape that closes over a post in place of a clutch to secure the earring to a pierced ear.

Onyx: A semiprecious variety of chalcedony quartz that is a used often in jewelry. When chalcedony is variegated with with spots or figures, or arranged in differently colored layers, it is called agate; and if by reason of the thickness, color, and arrangement of the layers it is suitable for being carved into cameos, it is called onyx. White Onyx, a semi-translucent white to slightly yellowish-white stone, was often used as a background in Victorian jewelry, especially for cameos. Black Onyx can appear Opaque or translucent. Although it occurs naturally in black, it is generally dyed black to improve its color. It has a hardness of between 6.5- 7 on the Mohs scale. In India, onyx necklaces are thought to cool feelings of love.

Opal: An amorphous non-crystalline variety of silica which is softer and less dense than quartz. Opals are known for their distinctive iridescent luminous qualities which are actually inclusions that can refract light in a rainbow of colors, called "fire", that change with the angle of observation (Dichroism). Opals contain a large amount of water and susceptible to cracking. Opal is the birthstone for October.

Opaque: A substance which is not transparent or translucent. i.e. a substance that does not allow light to pass through it.

Open-work: A setting that has open areas similar to filigree cut which may allow for viewing of the bottom of a stone. See Basket.

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

Optical Properties: A gemstone's inherent ability to interact with light. Color, dispersion and fluorescense are all optical properties.

Orb: A part of coronation regalia placed in the left hand of the Sovereign during the coronation as a symbol of Kingly power. It is a globe of gold encircled by a band edged with pearls and set with rubies, emeralds and sapphires, joined by a perpendicular band and surmounted by large amethyst above which is a jeweled cross.

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.

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

Orthoclase: A variety of feldspar, composed of potassium aluminum silicate, characterized by a monoclinic crystalline structure and found in igneous or granitic rock. Also called potash feldspar.

Ouvarovite: See uvarovite.

Oval Cut: A "fancy cut" diamond or stone in an elliptical shape like an oval.

Overlay: The upper layer of an overlay technique. The layer with the design.

Overlay Technique: A decorative technique involve two flat pieces of silver, or other metal. The top piece has a design cut out of it and is highly polished. The bottom layer is darkened with Oxidation to provide good contrast to the design on the top piece when the two pieces are soldered together.

Oxidation: A chemical process in which a metal, such as silver, is blackened as a reaction to Oxygen.

Oxide: A compound containing one oxygen atom per molecule. See also dioxide.

Oxidize: The act of combining with oxygen 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.