[Illustration: Sketch of a simple balance used to determine specific gravity of a gemstone. The operator places the gemstone in the upper pan (A), moves the weight (B) along the beam (C) until it balances perfectly, and notes the number at the weight’s position. He then transfers the gemstone to the lower pan (D), which is completely immersed in water, and moves the weight along the beam to restore balance. He notes the scale number at the new position and determines the specific gravity simply by dividing the first number by the difference between the two numbers. If the gemstone is large, the operator can use heavier sliding weights. (E).] Gemstones such as beryl and sapphire that depend on impurities for their color are said to be _allochromatic_; others, such as peridot and garnet, which are highly colored even when pure, are said to be _idiochromatic_. The color of a gem is further described according to its _hue_, _tint_, and _intensity_. Hue refers to the kind of color, such as red, yellow, green, etc.; tint refers to the lightness or darkness of the hue; and intensity refers to vividness or dullness. Throughout history, the most popular colored stones have been those with hues of red, green, or blue of dark tint and high intensity. [Illustration: A 43-carat albite from Burma (at left), 76-carat tourmaline from Brazil, and 30-carat wernerite from Burma exhibit a strong cat’s-eye effect because of reflection from inclusions in parallel arrangement within the stones. (Actual size.)] [Illustration: Asterism (star effect) is caused by parallel inclusions arranged in several directions related to the crystal structure of the gemstone. Two rays in the 175-carat, 6-rayed star garnet from Idaho (at left in photo) are weaker than the other four because of fewer inclusions in that direction. The 23-carat star orthoclase from Ceylon shows brightly all of its four possible rays. (Actual size.)] The effect of a gem on light may be more than the production of color. Several of the so-called phenomenal stones are prized for other effects. Holes, bubbles, and foreign particles, when properly aligned in parallel groupings, can produce interesting light effects. The play of colors of opal and labradorite, the _chatoyancy_ or silky sheen of tiger’s-eye and cat’s-eye, the _opalescence_ or pearly reflections of opal and moonstone, and the _asterism_ or star effect of rubies and sapphires are caused by the reaction of light to minute _inclusions_ or imperfections in the gemstone. When light passes into or through a gemstone with little or no interruption, the stone is said to be transparent, as opposed to a stone through which light passes with greater difficulty, and which is said to be either translucent or opaque, depending on the degree of light interruption.