What's so special about this gold stuff, anyway? I mean, it's expensive because it's rare but why do we use it in jewellery?
Gold has a couple of fairly unique properties that have made it attractive to jewellers throughout history. Prime among these is its resistance to corrosion. The only chemical that can dissolve or even tarnish pure gold is "royal water", a mixture so fiendish few of us are ever likely to encounter it and those who do will have other things to worry about than if it will stain their jewellery. (All super-masochistic claims about dipping pierced genitals in acid for pleasure will be scornfully disbelieved!)
So "gold is forever". Pure gold will keep its shine no matter what and if truly pure it will not release any nickel or other contaminants into the bodies of the allergic or hypersensitive.
Secondly, gold is extremely malleable and can be worked into amazingly fine detail. This is highly desirable for some types of fine filigree work and also means that a ring made out of 24 K gold can be easily opened and closed without special tools and without growing brittle and/or breaking as is the case with most harder alloys.
The disadvantages are clear. The price is high and in its pure form it's so soft it wears quickly from the purely mechanic rubbing of your skin and of other jewellery.
The common solution, however, is not without its flaws. "Cutting" the gold with cheaper metals can mean dramatic savings in material cost and highly improved resistance to wear but we must remember that it also changes the other special property of gold - its resistance to corrosion. Some chemicals in more-or-less popular use in body-piercing circles (Betadine, to be specific) will tarnish "gold" of as high as 18 K, though of course it is really the alloying metals that are affected. Also - and this is really the important bit - the more "impurities" you put in there, the greater the possibility that some of them will "escape" and get dissolved into your body. 14 K "gold" is frequently unsafe for those who have developed "nickel allergy" - a hypersensitivity to nickel. 18 K is usually safer, except for "white gold" where nickel is frequently used in dangerously high amounts in order to achieve the silvery color. There exist non-nickel "white gold" compounds but they're more expensive. Ask the dealer if you have the slightest fear that you may be sensitive to nickel - gold jewellery is too expensive for any experiments.
Nickel is a metallic element, number 28 in the periodic system. It is silvery in color and has a number of properties that make it attractive from a metallurgical point of view. It is quite corrosion-resistant and adheres very well to other metals, making it excellent for protective or decorative plating. It is also frequently used as an intermediate layer to improve adhesion between other metals, like when electroplating gold on silver, and as an alloying metal, like in many varieties of stainless steel and low-grade (less than 18 K) gold.
The problems when using nickel in jewellery stem from the one notable exception in the "quite corrosion-resistant" bit. It reacts very easily with a number of nitrogen compounds and unfortunately the amino-acids of our bodies are among them.
If you stick a nickel-plated needle through your skin, what happens is essentially that your body senses the intrusion and opens up the capillary walls in the surrounding area to let plasma and antibodies in to kill any bacteria and start repairing the damage.
Now, since you used proper sterile piercing procedure, there are no bacteria but some of the nitrogen compounds in these fluids will dissolve nickel from the surface of the needle and react with it. This is the danger, because the nickel that gets "bound" to the cells may change their composition sufficiently that your immune defense system will decide that they're no longer "you" and hence they're an infection and need to be fought. If this happens, the tissues swell up more, becoming a regular inflammation, and even more liquid stuff is sent in to fight the "nasties". Unfortunately, they will just dissolve more nickel and increase the problem.
The term "nickel allergy" is, strictly speaking, a misnomer since the problem is more of a hypersensitivity. Your immune defense system is simply doing its job. It's just a bit "overzealous". Nevertheless, the problem is real and can become very acute. Once the "allergy" is triggered, the sufferer will react to much lower concentrations of nickel than before. Jewellery that was previously safe may become useless. Some will react to the nickel in coins, railings, cutlery and other household items as it gets dissolved by their perspiration and permeates into the skin. Not to mention the nickel that can dissolve from stainless steel sinks into the dishwater and further into the skin of any person sticking his/her hands into the water.
So what can you do? First and foremost you must avoid nickel in jewellery that's inserted in fresh piercings or in moist places.
The issue is not so much if there is nickel in a certain alloy. What matters is that it STAY there. Gold, for example, has a tendency to "bind" nickel so that down to 18 K a small amount of nickel is usually "safe", except for "white" gold which often contains (and releases) too much nickel. When you get down to 14 K, the nickel released is often getting dangerously high, at least to those who are already sensitized. Stainless steels frequently contain nickel. The "hypoallergenic" varieties are frequently simply those that "bind" their nickel so thoroughly that little or none of it is released.
In extreme cases the sufferer can find even the most pure metals impossible to wear. Even 24 K gold (nominally 100%) can contain traces of nickel or other contaminations, but this is extremely rare. Most find a marked improvement in simply going from 14 K to 18 K. Other metals are now finding their way into the jewellery business, titanium and niobium rapidly gaining a well-deserved reputation for safety. The oxide layers on their surfaces are sufficient barriers against corrosion and wear and they also do not have nickel's tendency to bind into the cells of the body and trigger the immune defense system. Otherwise, inert plastics (Nylon, Teflon) work well. Some have reported successful experiments with "alien" organic objects like wood,
ivory or porcupine quills which don't contain anything that can dissolve into the body, but care should be taken with these since they can trigger your immune defense system in their own right, being (formerly) "living" materials themselves.
Other metals, like chromium, trigger similar effects like nickel, but less strongly so, in most cases. However, many nickel-free alloys contain very high amounts of chromium instead and hence they, too, may release enough chromium to trigger "allergic" reactions.
Source: Essay UK - http://www.essay.uk.com/coursework/metal-allergies.php