Jewelry and Gemstone Glossary of Terms

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Onyx

A variety of chalcedony quartz, often with parallel banding. When chalcedony is variegated it is called agate. Black onyx may be opaque or translucent and although it does occur naturally in black, it is usually dyed to darken its color. White onyx is a semi translucent white to whitish-yellow stone that was often used in Victorian jewelry for cameos. It has a hardness of between 6.5- 7 on the Mohs scale.

Natural black onyx is a brittle stone requiring care during cutting or carving.

Originally, almost all colors of chalcedony from white to dark brown and black were called onyx.  Today when we think of onyx we often preface the word with black to distinguish it from other varieties of onyx that come in white, reddish brown, brown and banded. A variety of onyx that is reddish brown with white and lighter reddish bands is known as sardonyx. 

Opal

Most opal is 50-65 million years old, dating back to the time of dinosaurs in the Cretaceous period. Opal formed as silica from decomposing rocks that mixed with ground water and formed a silica gel that hardened in underground cavities and fissures.

There are two distinct types of opal, common and precious. The way the silica particles form determines which type. In precious opal, silica particles are packed in regular rows and layers. Moving the stone causes light to diffract, or split, as it grazes the opal surface. This light diffusion shows iridescent flashes of green, blue, aqua and sometimes yellowish or red colors which are referred to as "fire". Opal is found in a wide range of colors including: green, blue, aqua, and pink. See opal varieties and boulder opal for more in depth discussions about this wonderful gemstone.

Opaque

In gemstone use, an opaque gem is one that does not allow light to pass through it, is not transparent or translucent.

Examples: lapis , turquoise, and malachite .

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.

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