Gold tarnishing?

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Why does gold turn black or why does gold not turn my finger black?

....Thanks for asking such an interesting question; I've searched for an answer for a while, and I am chagrinned to say that I don't have a conclusive answer for you. However, I do think that there are some possible explanations for the phenomenon you are experiencing. I looked through quite a bit of scientific literature, but I wasn't able to find much anything of use. So, hat in hand, I turned to jewellers. You will be happy to know that you are not the only one who has had this problem with gold jewellery. In fact, you only have the problem with your gold ring; other people have had this problem with all gold jewellery.

First off, the Jeweller's Associations of Australia and New Zealand have identified five reasons why gold jewellery may leave a black mark on skin.

1. Sometimes skin secretions and perspiration contain chemicals that react with the other metals in the gold alloy, leaving a residue. (The Jewellers Associations suggest that this can happen when changes occur in body chemistry, for example during pregnancy.)

2. Outside chemical influences, such as cosmetics, hairsprays and perfumes may sometimes be to blame for the smudge.

3. Air pollutants form industry may cause the smudge, which can rub off onto the skin when jewellery is worn.

4. Minute particles of dust or powder may be embedded in the skin which can result in extremely small particles of gold alloy being rubbed off and left on the skin.

5. The fifth and least likely cause may be allergy to gold or one of the other metals in the alloy, such as an allergy to nickel which is commonly used in fashion jewellery and in some white golds.

I also asked my local jeweller your question, and she suggested three (similar) reasons for the black mark.

1. Copper and silver in low karat gold alloys can tarnish, leaving a dark green-black mark.

2. Particulate matter can accumulate underneath a ring, where metal is tight against the skin, leaving a dark smudge.

3. Some people are allergic to the alloy metals in the ring, and can have an allergic reaction to the metal, resulting in a dark rash.

So, to get a better handle on your problem, you might to check the karat value of the gold in your ring and compare that to the rest of your jewllery. The karat (KT) value describes the purity of the metal. 24 KT gold is 100% pure gold, while 18KT is 75% gold and 25% alloy metals, and 12KT is 50% gold and 50% alloy metals. I don't think that jewellery can go below a karat value of 10KT (41.6% gold) and still be called gold in the US. My jeweller suggested that black marks resulting from tarnishing and rashes would only result from contact with low karat (e.g., 10, 12, and 14 karat) metals.