Most transparent minerals have, when pure, no color. Diamonds, being transparent, are no exception to this rule. Color is due to the presence of relatively small amounts of foreign substances, usually in metallic oxides. Iron oxide is one of the most common oxides, especially in diamonds. Sir William Crookes, the noted scientists, tells us that ash left when a diamond is burned in oxygen is largely iron oxide. The oxides that cause color in precious stones are thought by some to exist in very small particles, suspended, as it were, in the crystalline material. The particles are so small that they are invisible when viewed under a microscope. They cause a selective action upon white light that attempts to pass through the material, so that it comes out more or less colored. Light of certain colors is held up or absorbed, more than light of other colors, with the result that the light which emerges from the material is on longer white, as when it entered.



Iron as a colorant commonly produces yellow, brown, or sometimes red; a truly white diamond is very rare. Diamonds of a very light blue tint are scarce, but are more abundant that really snow-white stones. The presence or absence of color in a diamond exerts a very great effect on the value of a diamond. When goods are purchased in small or irregular lots that are less carefully graded, or when stones are taken into trade, the merchant needs to be able to discriminate the matter of color.

Aside from the diamonds which possess beautiful color, the tints in diamonds are pale and range from yellow and brown that is obvious, but not deep enough to be pretty. In such faint shades the presence of color cannot be seen by the average person, and at times not even by one trained, unless by direct comparison with stones known to be perfectly white.

It is the ability to detect these differences and determine how much color a stone carries, that successfully judging a diamond lies. The commercial value that goes along with the numerous grades may be learned in the trade, and the names that are applied. The most difficult part of grading a diamond is to definitely determine which class a certain stone belongs.



Artificial help in grading diamonds is one area that always needs covered. First of all, a diamond grader must not be color blind and also they cannot suffer from the lack of vivid perception. Practice and training is also necessary before great skill can be acquired. Given these prerequisites, how should one get the best results in grading?

First of all you need to have a good north light, unobstructed by any objects. There cannot be any colored surfaces near by to reflect tinted light. Second off, do not attempt to judge stones closely except in the middle of the day, between 10 A.M. and 2 P.M. Very erroneous results may result if this precaution is neglected. Dark and gloomy days should also be avoided. You need to have lots of good neutral lights to make fine comparisons.

It is nearly impossible to make fine distinctions by artificial light. A gem with distinct tints of color will appear white by some kinds of artificial lights. If you have to make a decision in the evening, you may use a tungsten filament light. A tungsten filament light gives the nearest approach to daylight of any of the kinds of artificial light. It is not a good idea to attempt to grade stones in unfamiliar surroundings. If at all possible try to use the same place every time while studying the color of stones.

An essential is to have with you for comparison, stones whose color you are sure of. Rough distinctions can be made by people who are experienced, without the aid of a comparison. When it comes down to making a comparison between two very fine stones the absence of an aid makes it almost impossible.