Allergy caused by metals are like any other allergy. It is occur as a result of an “invader”, not familiar molecules wondering through the lymph nodes. The most common allergy among metals is nickel when it's been used in high amount in alloys to make cheap jewellery.
It is difficult to tell the difference between an allergic reaction and an infection, both have similar side effects such as: irritation, redness, swelling, burning, itching. However, allergic reaction won’t improve after taking care of the infected area with the right treatment.
Green stains are not an allergic reaction to metals, read more here.
If it's the first jewellery for your new piercing, It is always recommended to start with metals known as “hypoallergenic” such as Surgical Steel (316L) and Titanium. Choosing these metals for a new piercing is a smart choice as they are biocompatible- The body doesn’t recognize them as a foreign objects and reacts to them.
Other metals such as silver and gold contain in their alloys metals such as copper that may cause a reaction, therefore they are not hypoallergenic.
Surgical Steel (316L) The most common stainless steel to use in piercing jewellery, body implants and where a metal with minimal contamination is required. The letter L refers to “low Carbon”, it has trace amount of nickel and doesn’t react with body fluids.
Titanium Consider to be the top choice for long term piercing jewellery, it is strong, light in weight, like 316L it is also used for body implants and contains just a tiny amount of nickel.
Platinum The most valuable metal of all, it has an attractive shiny look and can be a descent replacement for people who are sensitive to silver and gold.
Sterling Silver Contain 92.5 silver and 7.5% other metals, the most common is copper or zinc. Developing reaction to silver is not common as it contains only small amount of other metals and it’s nickel free. But, if you do get irritated, it is probably a copper allergy and the solution for it is pure silver jewellery which contains 99% silver.
Gold Is an alloy of silver, copper and a little zinc. It is pure form that itself should not cause allergic reaction. However if you do get reaction it is a result of the copper amount in the alloy. To try to avoid cheap alloys, check the jewellery quality and make sure you pay as gold value.
A tiny Tip, gold is a soft metal, thus, when considering getting a solid gold piercing it should be between 14k to 18k. more karats can make the jewellery too soft and bendy.
Brass An alloy of copper and zinc. It can be produced in a range of colours, from deep red to golden yellow depends on the metal's polish process. Brass is nickel-free, yet people who reacts to brass jewellery, are probably allergic to copper.
PVD Coating Are popular to colour surgical steel piercing. The process of making PVD is environmental friendly, it doesn’t contain harsh chemicals and doesn’t create toxic by products. Concern this material to hypoallergenic.