Gemstones — The Big Four
In the beginning, before modern gemology, there were only four gemstones and they were named for their color. White (clear) were diamonds, red were rubies, blue were sapphires and green were emeralds. Then man became a little more educated and he found that all stones of the same color were not the same crystal shape and/or composition. Oops, maybe some of our previous stones were not what we thought they were.
Now on to the actual topic. There are two types of faceted gemstones: White and Colored. The only white gemstone is diamond; all others are colored stones. So a crystal clear piece of quartz is a colored gemstone. The terms precious and semi-precious should not be used as several of the colored stones are more valuable, and rarer, than diamonds. Also, all diamonds, no matter what color are white stones. Looking at the first of the big four:
Diamond — Diamond is a pure form of carbon, with no contaminants. Diamond gems range from internally flawless perfectly clear (D IF) to slightly yellow (Z). The colors of diamond are graded from D to Z, getting slightly more yellow across the range. Brilliant yellow diamonds are called fancy diamonds (along with all the other colors). Diamonds are mined in Africa, Australia, Canada and Russia. Diamonds are also produced artificially using a high pressure high temperature process (HPHT).
Rubies are red; sapphires are any other color. Both of these are the same mineral, corundum, which is aluminum oxide. For rubies, the color is produced by the presence of chromium in the crystal matrix. The presence of chromium can be easily seen using a Chelsea filter. Sapphires come in just about every other color including white (clear).
Emeralds are the green variety of the mineral beryl. Chemically this is an aluminum silicate with a hexagonal crystal. The color is caused by the presence of chromium in the crystal matrix. (Yes, in ruby, this causes red.) The way the light is reflected is different from ruby, so we get the green instead. However, the presence of chromium is again easily detected with the Chelsea filter.
Diamonds, rubies and sapphires should be flawless and pure of color. If there are inclusions and/or fractures/feathers, etc. in the stone then the value is lessened. However, if you do see inclusions, that will usually mean that the stone is natural and not synthetic. Emeralds, however, are filled with inclusions (called the garden) and should not be clear. In fact, the inclusion of calcite or pyrite helps determine which mine in Columbia produced the emerald. Emeralds are commonly oiled to hide some of the fractures.
If you are unsure about the identity of your gemstone, consult a gemologist. Most jewelers do not have the equipment and/or knowledge to properly identify gemstones. With the modern ability to identify gems and separate natural from synthetic, failure to make the correct identification is unacceptable. For more information, consult the International Gem Society or the Gemological Institute of America.