Mary Andreesen, age 15, of Reno, Nov., for her question:
WHAT MAKES A GEMSTONE?
A gemstone is a mineral that is treasured for its beauty and durability. A large number of minerals have been used as gems. Here's what sakes a gemstone: the beauty of the stone itself, its rarity, its hardness and toughness and the skill with which it has been cut and polished.
Stones such as diamonds, rubies and emeralds represent one of the greatest concentrations of money value. A few diamonds of good quality say be worth a fortune and are easily negotiable in most parts of the world.
The beauty of gems depends to a large extent on their optical properties. The most important optical properties are the degree of refraction and color. Other properties include fire, the display of prismatic colors; dichroism, the ability of some gemstones to present taro different colors when viewed in different directions; and transparency.
Diamonds are highly prized because of their fire and brilliance. Rubies and emeralds are prized because of the intensity and beauty of their colors. And star sapphires and star rubies are prized because of the star effect, known as asterism, as well as their color.
In certain gemstones, notably opals, brilliant areas of color can be seen within the stone. These areas change in hue and size as the stone is moved. This phenomenon, known as play of color, differs from fire and is caused by interference and reflection of the light by tiny irregularities and cracks inside the stone.
Opals also exhibit milky or smoky reflections from within the gem.
Gems that are fibrous in structure show irregular interior reflections similar to those seen on watered or moire silk. This optical property, which is called chatoyancy, is exhibited by several gems, notably the tiger's eye and cat's eye.
Another important property of gemstones is called luster.
The shaping and polishing of gem materials to enhance their beauty and, in some cases, to remove imperfections, is performed by expert workers known as lapidaries.
Gems are shaped entirely by being ground on abrasive wheels or revolving abrasive disks. For minerals that are no harder than quartz, natural sandstone wheels are sometimes used, but for the harder stones, such as rubies and sapphires, synthetic grinding wheels of cemented Carborundum must be used.
The first step in the cutting of a gem is to saw it roughly to shape. Thin abrasive disks or metal disks charged with powdered diamond or other abrasives are used in this process.
The stone to be shaped is cemented to the end of a wooden stick called a dop and is held against the revolving wheel or lap with the aid of a supporting block placed adjacent to the wheel. This supporting block contains a number of holes in which the end of the dop can be rested.
By changing the dop from one hold to another, the lapidary is able to control the angle of the facet, or face, being ground.