925 sterling silver, which is also just called sterling silver, is an alloy of silver containing 92.5% by weight of silver and 7.5% by weight of other metals, usually copper, but sometimes also nickel or zinc. Manufacturers of silver metal products used by jewelry artists, such as silver wire and chain, do not normally list the other metals which make up the 7.5%. Other metals are added because fine silver, which is 99.9 % pure is too soft to hold its shape for many types of jewelry work.
Unlike gold plated jewelry, which has a very thin layer of gold over a base metal that can wear off, gold filled items have a thick layer of solid gold bonded to a base metal layer. 14k gold filled items are manufactured with a government regulated thickness of real, 14k solid gold on all the outer surfaces. Gold filled items are required to have an amount of solid gold that is at least 1/20th of the total weight making the thickness roughly 100 times thicker than gold plating. The base metal core of gold filled items is often made of brass to match the gold color, but it can also be rhodium or sterling silver or other metals. Manufacturers of 14k gold filled metal products used by jewelry artists, such as jewelry wire and chain, do not normally list the base metals used, but the layer of solid gold covering the base metal is thick enough that it does not wear off or tarnish. In their work, jewelry artists may manipulate the rough product, such as cutting jewelry wire into sections or hammering or cutting metal sheets, which may expose a small portion of the base metal. 14k gold filled jewelry is a wonderful, lower priced alternative to solid gold pieces.
All colors of 14k gold contain 58.3% pure gold combined with other metals to strengthen the gold, which is too soft to be used in jewelry making on its own. The different colors come from the other metals used. The most commonly used metals to alloy with gold are silver, copper and zinc. Rose gold contains more copper for the rosy hue and yellow gold has more of the white metals of silver or zinc. Because of the higher copper content (copper is a reactive metal), rose gold can patina under certain conditions.
White gold contains other metals, such as palladium and nickel to give it a more white color and it is usually then plated with the whiter metal rhodium since pure gold is naturally yellow in color.
10k gold is 41.7% pure
14k gold is 58.3% pure
18k gold is 75% pure
24k gold is 99.9+% pure, but too soft for jewelry making
NOTE that gold is never consider hypoallergenic, but the more pure it is the less likely it is to irritate.
Copper is a relatively inexpensive metal that has wonderful properties for jewelry making such a malleability, a skin flattering rosy gold color, ability to hold a patina (such as antiquing or verdigris) and exceptional natural anti-microbial properties. Some people even wear copper for its reputed therapeutic benefits. Raw copper is pure copper that has not been mixed with or treated with anything. Raw copper is refined to more than 99.9% pure because a chief usage of copper is for electrical conductivity which requires a high purity level.
Copper is a reactive metal which will develop a patina over time due to humidity and moisture, an individual's body chemistry, salt air or pollution. This may happen quickly. The tarnish may transfer to the skin, but is harmless. Copper should not be worn in the shower, while swimming or for sports.
Some commercially available copper jewelry findings, such as clasps, beads and jump rings, are coated with an anti-tarnish coating. Some manufactured copper jewelry findings may contain some nickel used to harden the soft copper. Soldered link copper chains are soldered with a silver color solder and then flash plated with more copper to bond with and cover the solder color.
ANY alloyed (mixed) metal can cause allergies in some people. Nickel is a common irritant.
Metals that are generally safe are titanium, platinum, brass (copper and zinc), bronze (copper and tin), niobium, argentium silver (93% silver plus copper and germanium to keep it from tarnishing), fine silver (99% pure silver) and pewter.
Any special coatings, patinas, anti-tarnish coatings, etc may cause reactions in some people.