This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
1 karat = 1/24th of the alloy is pure gold
Only solid gold of at least 14 karat (58.3% gold) is appropriate for
body jewelry. Some piercers prefer to use only 18k gold (75% gold) in
Gold-filled and gold-plated or jewelry is not appropriate. Gold
plating is very thin and can wear away quickly with the friction to
which body jewelry is exposed. Bending the jewelry after it is plated
will cause the plating to fracture and chip.
Some people are sensitive to the metals present in karat gold, namely
nickel, silver, zinc and copper. White gold often causes more adverse
reactions than yellow gold because a high amount of nickel is used to
yield the white color. Many body jewelry manufacturers now use
nickel-free gold alloys. White gold alloyed with palladium, an inert
metal of the platinum group, instead of nickel is less likely to cause
a metal sensitivity. Some people who cannot wear steel alloys can
wear 18k white gold palladium alloys.
Green or pink gold should not be worn because of the higher
concentration of copper and zinc used to produce the color.
Some people’s perspiration is acidic enough to corrode the non-gold
alloys in karat gold, evident by blackening of the jewelry and
discoloration of the surrounding skin. Over a long period of time gold
jewelry exposed to urine may acquire a rough, dull surface. The acids
found in urine leach into the surface and dries to form a hard
crust. Buffing or polishing the jewelry will remove this
build-up. Daily cleaning of the jewelry will prevent this build-up.
Gold jewelry will often become discolored when exposed to povidone
iodine. Gold jewelry may become discolored when steam autoclaved; the
indicators on sterilization packaging and tape appear to cause
discoloration. Discolored jewelry can easily be repolished with a
soft buffing cloth.
Jewelry that has undergone surface depletion, or pickling, will be
less likely to discolor. Pickling is a process of heating the jewelry
or submersing it in a weak acid which removes oxidation of non-gold
alloys from the surface of the metal.