Unit 2: Composition of Earth Chapter 4: Minerals     Minerals Gems are minerals that are valued for their rarity and beauty. Some of the world's most precious gems are diamonds. Not all diamonds are valued alike, however-some are more expensive than others. Why? Diamonds may differ widely in their color, cut, clarity, and carat weight. Color The color of a diamond is caused largely by its chemical composition. The presence of nitrogen, for example, may cause a diamond to appear yellow or brown, while boron produces a blue diamond. Sunlight, which contains ultraviolet rays, can cause a yellowish stone to appear more blue. In general, a colorless diamond is more valued than a yellow diamond, and a blue diamond is the rarest and most expensive of all. Cut The cut of a diamond also determines its ultimate value. When viewed from above, a brilliant-cut diamond appears round. Viewed from the side, it has a flat top, known as a table, and an upper portion, called a crown. The crown slopes to a wider middle, which jewelers call a girdle. The lower portion of the cut diamond, referred to as a pavilion, tapers almost to a point. Each brilliant-cut diamond has 58 polished flat surfaces called facets. These facets act as mirrors. When properly cut, 80 percent of the light that enters a diamond does not pass through but is bounced back and forth within the gem and eventually released from the top. The sparkle or "fire" of a diamond is caused by this refraction of light. Refraction of light also causes the rainbow of colors that flash from diamonds. This occurs because different wavelengths of light are bent as they enter the diamond and white light is separated into its component colors. A diamond must be cut to very exact proportions for this refraction to occur. The table must be 53 percent of the width of the girdle, the crown must be 16.2 percent, and the pavilion must be 43.1 percent. The angle at the bottom of the gem must be 98.5°, the angle between the girdle and the crown must be 34.5°, and the angle between the girdle and the pavilion must be 40.25°. Any deviation from these ideal proportions results in some light being lost through the bottom of the diamond. The cut of a diamond is not the only factor that affects its sparkle. Diamonds reflect more light than most other transparent substances. A polished surface of a diamond, for instance, reflects 17 percent of the light that hits it, while a flat piece of glass only reflects 5 percent. LINK-UP: Find out more information on cutting diamonds. Flaws and Other Factors The value of a diamond is also dependent on its clarity, which refers to the presence of flaws and other impurities in the diamond. A flawed diamond has less commercial value than a flawless diamond. From a scientific point of view, however, flawed diamonds are highly valued because they form deep within Earth and thus give scientists information on the processes that occur at depths of 140 to 190 km. Lastly, the value of a diamond is dependent on its carat weight. The higher the carat weight, the greater the cost of the diamond. Carat is the unit used to measure most precious gems. It is related density, not size. Different gems have different densities. Thus, a one-carat sapphire is smaller than a one-carat diamond because sapphire is denser than diamond Activity Visit a jewelry store to examine diamonds of different values. Compare and contrast the gems. Prepare questions beforehand to ask the sales clerk about the price and the quality of the diamonds. Share your results with the class.