Old European cut - a brilliant cut variation pre-dating today's round brilliant diamond. There is usually 58 facets including a small table and a deep crown and pavilion.
Old Mine cut - a brilliant cut diamond pre-dating the Old European cut diamond. It has a cushion shaped girdle profile, a small table facet and a deep crown and pavilion.
Opaque - when light rays cannot penetrate an object.
Organic Gem - materials formed from living or once-living organisms. Examples of this would be amber, pearl, ivory, coral, etc.
Overlay - when one sheet of metal is soldered onto another.
Overtone - a tint of color that appears on the body color (main color) of a pearl. An example would be a white pearl that has a tint of pink in it.