World Book Trivia Challenge Advanced-11/26-gemstones

Jewelers cut lesser-quality peridots into a rounded style called cabochon, or polish them with abrasives in a process called tumbling.

Lapidaries (gem cutters) cut most gems by grinding away material until the desired shape is reached.

Hardness, transparency, and index of refraction determine the way a gem is cut. There are two main styles of cut gems: (1) faceted gems, which have many tiny polished sides called facets, and (2) cabochons «KAB uh shawnz», which are rounded, polished stones.

The beauty and rarity of natural gems has caused a constant demand for them. But their costliness has prevented many people from owning them. Thus, synthetic and imitation gems have become the foundation of a major industry. The basis of some inexpensive imitations is a soft glass called paste.

Opal consists of water and a mineral called silica, the most common ingredient in sand. Precious opal forms underground when water flows through cracks and spaces in the rocks. The water slowly evaporates, depositing tiny spheres of silica. If the spheres are the right size and in a uniform structure, they diffract (bend) light. Light diffraction within an opal removes some of the colors in light and strengthens others. This bending of light produces the play of color seen in gem-quality opal.

The custom of wearing a stone that represented a person's zodiac sign probably originated in Germany or Poland in the 1700's. It quickly became widespread. Through the years, jewelers and gem dealers have added new gemstones, such as tanzanite for December, to the traditional birthstone list.

Cat's-eye is a gem that produces a thin streak of white light across the top of the stone when the gem is cut a certain way. The streak resembles the pupil of a cat's eye. The streak seems to change position as the stone is moved. This change is known as the chatoyant effect and is caused by light reflecting from tiny hollow channels within the stone.

Cameo is an engraved gem. Most cameos are made of stones or shells that are stratified—that is, they are formed in layers. Artists cut a design into the top layer to produce a beautiful color contrast with the next layer.

In 1970, the General Electric Company produced the first synthetic diamonds of gem quality and size. Scientists use these diamonds to research new uses for diamonds. For example, researchers have found that adding small amounts of the chemical element boron to synthetic diamonds turns them into semiconductors. Semiconductors are materials with special electrical properties.

Gem minerals are natural materials made into gemstones. To identify such minerals, experts sometimes must use chemical tests or X rays. But gem minerals can usually be identified by such properties as (1) crystal shape, (2) color, (3) index of refraction, (4) cleavage, (5) hardness, and (6) specific gravity.