Obsidian
Obsidian (also called Apache tears) is a volcanic glass that is usually black, but is occasionally red, brown, gray, green (rare), dark with "snowflakes," or even clear. This glassy, lustrous mineral is found in lava flows, and obsidian stones can be massive. Obsidian is formed when viscous lava (from volcanos) cools rapidly. Most obsidian is 70 percent silica. Obsidian has a hardness of 5 and a specific gravity of 2.35. The pin above is Mahogany (brown) obsidian.

Oiling
Oiling is a process of applying mineral oil to a stone in order to enhance it and mask inclusions, make them more transparent, and darken their color. Emeralds are frequently oiled to mask their many inclusions.

Old Mine Cut
Old mine cut is a term that refers to a brilliant cut in which the stone is cushion-shaped and has a high crown (the upper part of a gemstone).

Old Rock Turquois
Old rock turquoise is an old Persian (Iranian) turquoise term for very high quality turquoise (sky blue, veinless turquoise that retains it color).

Olive
Olive is a term that refers to a bead that is olive shaped (elongated). This term is mostly used in the USA.

Onyx
Onyx is a semi-precious stone that is black and white, generally arranged in layers. It is a form of agate with parallel banding. This structure lends itself to cameo making. Onyx is a species of chalcedony (microcrystalline quartz).

Opals
Opals are semi-precious stones that are luminous and iridescent, frequently with inclusions of many colors ("fire"). Opal is a mineral composed of noncrystalline (amorphous) silica (and some water) and is a species of quartz. There are three major types of opals: common opal, opalescent precious opal (white or black, with a rainbow-like iridescence caused by tiny crystals of cristobalite), and fire opal (a milky stone that is fiery orange to red in color with no opalescence). Contra luz opals are transparent opals that show a brilliant play of iridescence only when light shines through the stone. Many opals have a high water content - they can dry out and crack if they are not cared for well (opals should be stored in damp cotton wool). Some opals are treated with oil, wax or resin to enhance their finish. Opals have a hardness of 5.5 to 6.5 and a specific gravity of 1.98-2.50. Opals are found in many places worldwide, including Kenya, Czechoslovakia, Brazil, Peru, Honduras, Mexico, Canada, and the USA -- but Australia has a tremendous variety of beautiful opals.

Opal Doublet
An opal doublet is a manufactured stone that is composed of two thin layers that are glued together. A thin layer of opal is glued on top of another mineral (usually a black onyx or ironstone, which enhances the stone's color), producing a stone that is less expensive than a solid opal. Doublets must be cleaned very carefully

Opalescence
Opalescence is a milky white/blue type of iridescence.

Opal Glass
Opal glass is a milky white glass that mimics opals.

Opal Triplet
An opal triplet is a manufactured stone that is composed of three thin layers that are glued together. A thin layer of opal is sandwiched between a layer of clear quartz and a layer of either obsidian or ironstone. The clear quartz is the top layer, making the gem harder (and less susceptible to scratches). An opal triplet is an opal doublet with a quartz layer on top. Triplets must be cleaned very carefully.

Opaque
Opaque means blocking the passage of light (as opposed to translucent or transparent).

Open-Ended Necklace
An open-ended necklace has no clasp; it is worn by tying the ends together around the neck. Open-ended necklaces usually have ornaments, like beads or tassels, at the ends.

Opera-Length
An opera-length necklace is a single strand that is from 30 to 35 inches (60 to 90 cm) long. Opera-length generally refers to a string of pearls that hangs to the breastbone.

Operculum
The operculum is part of many shelled animals - it is the calcified, disc-shaped "trap door" that opens and closes to protect the animal inside its shell. The operculum from a species of sea snail called the Turban Shell (Turbo petholatus, found in the South Seas north of Australia) is eye-like with a natural cabochon shape and is used in jewelry. This jewelry was popular in Victorian Era Britain. Operculum is also called Pacific Cat's-eye.

Ora
Ora was a costume jewelry company that was originally called Agnini & Singer; it was founded by Oreste Agnini and Ralph Singer (born Raffaele Cantaluppi) in Chicago, Illinois in 1921. They supplied the Eisenberg company with its early buttons, brooches, and dress clips. The tradename "Ora" was not adopted until the late 40s. Early pieces are unsigned. Mr. Agnini retired in 1953 and Ralph Singer bought his half of the company. The Company then became "The Ralph Singer Company" and continued using the "Ora" trademark, which is a combination of the names "Oreste" and "Ralph." When Ralph Singer died in 1963, Raymond Pausback became a partner, running the company, and eventually buying it. When he retired in 1984, he sold the company to Stanford Smith, who ran the company until his death. His son, Stanford Smith Jr. then ran the company. They still manufacture costume jewelry in Chicago, and still uses the "Ora" trademark. They now sell jewelry online. The Ora earring above is an old piece that is studded with clear rhinestones.

Ormolu
Ormolu (meaning "ground gold" in French) is an alloy of the metals copper, tin and zinc that is used to imitate gold. Ormolu can also be cast bronze or brass that is plated (gilded) with a gold and mercury amalgam, giving it a gold-like look. Ormolu is used in frames, chandeliers, candlesticks, and furniture ornamentation. It was very popular in Georgian and early Victorian design. Ormulu can now also refer to any gold-like metal used as decoration. Ormolu is also called bronze doré or mosaic gold.

Orthoceras
An extinct member of the cephalopod class with a long, straight, conical shell which could grow as long as 60 feet. The interior is divided into numerous chambers. These invertebrates swam the ocean by using a jet propulsion type system that expelled water from the chambers thus pushing them forward. They are related to the modern day squid, cuttlefish, and octopus and date back 350 million years ago. Fossilized cephalopods are found in the Sahara Desert, Morocco. Also, see ammonites.

Ouro Verde
Ouro verde (meaning "green gold" in Portuguese) is a type of quartz crystal found in Brazil. This transparent stone is always irradiated (to give it its pale, golden-green color).

Oxidation
Oxidation is a chemical process in which oxygen atoms bond to atoms of a material (like a metal) and electrons are transferred from the oxided material to the reduced material. Iron oxidizes when exposed to air and moisture, forming iron oxide (rust). Silver oxidizes (tarnishes, turing the surface black) when it is exposed to hydrogen sulfide in the air (forming Ag2S, silver sulfide).