Pure gold (fine gold) is softer than pure silver but harder than tin. Its beauty and luster are unmatched by any alloyed golds. The extreme malleability, ductility, and softness of pure gold make it practically useless for jewelry applications.
The addition of alloying elements (other metals) to gold are used to increase the toughness and hardness of the metal. While almost any metal can be alloyed (melted) with gold, only certain metals will not dramatically change the color or make the metal brittle. The addition of indium, for instance, turns gold purple and gives gold the workability of glass.
Over time, certain percentages of gold have become legally recognized 'karats'. The karat indicates the amount of gold as a percentage of the total, i.e. 24 karat is 100 percent gold. Thus 14 karat is 14/24's gold or 58-1/3 percent gold. Gold standards vary around the world. In the United States, 18, 14, and 10 karat gold are the only karats allowed to be sold as karated gold.
In karated gold, there is a balance of metals in the non-gold percentage. These metals provide the various colors and hardness of karated golds. Typical alloying elements and their color effect are:
Examples of the compositions of different colors are:
Yellow Gold, copper, silver, zinc
White Gold, copper, nickel, zinc
Red Gold, copper
Green Gold, silver
You may think that faulty manufacturing or underkarating might be the problem when a ring turns, blackening or discoloring the skin and clothing, or the jewelry itself. However, that is not the case. The most common reason is metallic abrasion, caused by makeup on skin or clothing. Cosmetics often contain compounds harder than the jewelry itself, which wear or rub off very tiny particles. Very finely divided metal always appears black rather than metallic, so it looks like a jet-black dust. When this dust comes into contact with absorbent surfaces such as skin or clothing, it sticks, forming a black smudge. To prevent this, you should try switching cosmetics. If this is not possible, we recommend that you remove rings and other jewelry while applying them, and clean skin areas in contact with jewelry with soap and water.
Another cause is actual corrosion of the metals. Gold itself does not corrode, but its primary alloys of silver or copper will do so—forming very dark chemical compounds—under moist or wet conditions. When you perspire, fats and fatty acids released can cause corrosion of 14-karat gold, especially when exposed to warmth and air. This problem can be worse in seacoast and semitropical areas, where chlorides combine with perspiration to form a corrosive element that discolors skin. Smog fumes gradually attack jewelry and are evident as a tarnish that rubs off on the skin.
On several occasions I have come across the suggestion that a general iron deficiency or anemia can be the cause of this black smudging;. I am aware of no studies done to date on this correlation. Specifically, the correlation between a persons iron levels and their ph levels. I suggest that there is a direct and identical correlation between a persons iron level and their ph level. That is to say as their iron level decreases so does their ph levels. Their body chemistry becomes more acidic. Therefore, the cause of the discoloration would be the increase of acidity, as outlined above, and not the iron deficiency itself.
We suggest that you remove jewelry often and use an absorbent powder, free of abrasives, on skin that comes into contact with jewelry. Even the design of jewelry can be an influence. Wide shanks have more surface area to contact abrasives or corrosives. Concave surfaces inside a shank form collection points that trap moisture and contaminants, also causing a type of dermatitis. You should remove all rings before using soaps, cleaning compounds or detergents, and clean your jewelry frequently. As well as solving the problem, you'll be amazed at how much better your jewelry looks! As always, have your jewelry professionally inspected 3-4 times a year. A minor repair can become a major replacement if left unchecked.
Okay, first things first. Pure gold is bright yellow. Not only do we alloy gold to provide strength we can alter it's color. Nickel is one the metals used in the alloy to whiten the gold by most manufacturers.
To over simplify, consider the pure gold, yellow paint. It doesn't matter how much white paint you add to it, it will never be white. So, in jewelry, we rhodium plate the item to give it a very bright white appearance.
Your jewelry is not 'turning yellow', the rhodium plating has been worn through and you're seeing the true base color. This is easily corrected by a trip to your favorite jeweler and after repolishing and rhodium plating, your jewelry will look splendiferous again! Today, there are more alloys used that provide a bright white finished piece of white gold jewelry. Check with your favorite jeweler to discuss what is available.