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.