Old mine cut: See Mine Cut.
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. See Onyx Rings: 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.
Opera Length: A necklace which is 48 to 90 inches long.
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.
Oriental Pearl: A pearl that has formed naturally with no human intervention.
Ouvarovite: See uvarovite.
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.
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.