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).

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).

OPAL
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 firey 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). Opals have a hardness of 5.5 to 6.5 and a specific gravity of 1.98-2.50. Opals are found in many placees worldwide, 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 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.

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).