Gold is one of the world's rarest metals.  Gold does not rust or corrode and it lasts forever.  One ounce of gold can be stretched in a wire that can cover 5 miles!  That is because gold is so workable and soft.  Understanding the different qualities of Gold in jewelry will make it clearer to you where the differences lie in the quality and the cost.  24 karat pure gold is too soft for everyday use, that is why alloys (other metals) are added.  Alloys give gold durability and hardness.  Alloys also can give gold a different color, such as adding copper to make a pinkish gold. Adjusting the proportions of the alloys (coloring agents) provides the array of colors in the gold market.

Alloys Used

White Gold - gold, copper, nickel, zinc.
Yellow Gold - gold, copper, silver, zinc.
Rose Gold  - copper, gold, silver, zinc.
Green Gold - gold, silver, copper, zinc.

Gold is always measured in Karats. (Carat spelled with a "C" to avoid confusion is used as a measurement of the weight of Gemstones.)   Karat refers to the fineness of the Gold, the proportion of Gold to the amount of alloy used.   24 karat( kt or K) gold is pure gold.  Imagine a pie divided into 24 equal slices.  24kt means all 24 parts are 100% pure gold.  18kt would be 18 equal pieces are pure gold the other 6 are other metal alloy.  And so on. In the United States Gold content is indicated by a Karat stamping on the piece of jewelry.  14kt - would mean 58.3% pure Gold and 41.5% alloys.

Why Does Gold Discolor My Skin?

Pure gold does not tarnish but the alloys in it sometimes do, this is what causes that black smudge.    The most common reason for this with women is cosmetics.  Cosmetics often contain compounds that are harder than the jewelry itself.  This causes to rub off tiny particles of metal that looks black, making a black smudge on the finger.  Perspiration can cause this problem too. Solution to this is to remove jewelry often and wash  your skin with soap and water. 

Keep jewelry clean and wipe with a soft cloth to remove any tarnish build up.  Sometimes switching to a higher gold content (14kt to 18kt) will solve the problem.   Switch to a different manufacturer of the jewelry, as different manufacturers use different ratios of alloys in their gold jewelry and this could be the problem.
 
Remember - Remove your rings before using soaps, cleaning solutions or detergents.   Always take your rings off when swimming in chlorinated pools.  Clean your rings frequently.   You may solve your problem and you will be amazed how much better your rings will look!