Skin reactions to jewelry are most commonly caused by a nickel allergy.
Silver jewelry can turn yellow due to a chemical reaction with sulfur compounds in the air or on the skin. This reaction forms silver sulfide, which gives the jewelry a yellowish tint. Regular cleaning and storing in airtight containers can help prevent this discoloration.
Jewelry changing color can happen because of different reasons. Chemicals, lotions, chlorine in water, all of these can change the color of some metals like white gold. If your skin is changing colors you may be having a reaction to one of the alloys in the metal your wearing. See if this reaction happens with all metals that you wear to try to pin point your allergic reaction.
If it is low carat gold (10 or so), the alloy metals might react to your skin. Anything but 24k will have some alloy in it, and jewelry is rarely made of 24k, since it is so soft. If you are highly sensitive to the alloys, even a small amount might cause a reaction. Furthermore, you sure your jewelry is really solid gold, and not plated? Plated jewelry will often react with the skin as you have described.
Jewelry that is uncomfortable or jewelry that you make have an allergic reaction to should not be worn.
Fake jewelry changes color because the plating or paint used on the metal can wear off over time, exposing the base metal underneath. This can cause the jewelry to tarnish or appear to change color. Additionally, cheap materials used in fake jewelry can react with the wearer's skin chemistry or environment, leading to discoloration.
http://www.helium.com/items/597008-why-do-some-jewelry-metals-turn-skin-green
It is best to by hypo-allergenic metal body jewelry for sensitive skin.
Stop wearing costume jewelry.
The main cause of green discoloration on hands is a reaction between certain metals (like copper or brass) in jewelry and the acids in sweat. This reaction can result in the skin turning green due to the formation of copper salts. Proper cleaning and maintenance of jewelry can help prevent this discoloration.
Yes
I think it is bad for skin
Yes, copper can be used for jewelry-making. It is commonly used for creating unique and affordable pieces. However, copper jewelry may tarnish over time and can cause skin discoloration in some people due to its reaction with sweat and oils.