Tracy Ryan, age 9, of Des Moines, Iowa, for her question:
HOW DO ROCKS BECOME GEMS?
A gem is a selected stone which, when cut and polished, becomes an important part of a piece of jewelry. Sometimes these gems are called precious stones since most are rare mineral crystals found in nature.
Pearls, some coral and amber, are classified as gems although they have animal backgrounds.
Few gems that are taken from the earth come out looking beautiful. In most cases they must be cut, shaped and polished by gem cutters who bring out the beauty in the stones.
Brilliance of a gem is due to the amount of light it reflects. Some of the light is reflected from the surface of the crystal while some enters the crystal, is bent or , refracted when it strikes an interior surface, and is then reflected out from the front again. The greater the amount of light reflected from the interior, the greater the brilliance of the gem.
There are about 2,000 minerals found in the earth, but only 16 of them are used as gems.
Emeralds and aquamarines are varieties of the mineral beryl. Rubies and sapphires are types of corundum. Agate, amethyst and onyx are quartz. The 13 remaining gem materials include diamond, garnet, jade, opal, topaz, turquoise, zircon, moonstone, lapis'lazuli, peridot, spinel and tourmaline.
What makes a rock a gem? Its beauty is what sets it apart, and the beauty of a gem is due to its color and brilliance. Gem materials are usually found in nature as colorless, pure and transparent crystals. Such a mineral is the diamond. Occasionally they are found with some impurities which cause them to appear colored. When the impurities are evenly distributed throughout the crystal a rare event a gem such as a ruby, emerald or sapphire results.
A diamond reflects more light than any other gem. It can be cut by an expert in a number of different ways. It is the hardest mineral known, yet it contains the same chemical element found in coal, charcoal and graphite.
Because it is the hardest substance known, the diamond is also used for cutting, grinding and boring into hard metals and other hard substances.
As you've probably already guessed, a diamond can be cut only by other diamonds.