Fine metals and the sliding scale of karats: 10k, 12k, 14k, 16k, 18k, 24k. You can usually tell the level of karat by the color of the gold. Lighter the gold, less in karat. Higher the karat, the deeper the yellow it becomes. 10k is less gold content and higher ratio of metal alloy. 24k is pure gold all way and nothing mixed with any metal alloy, very brittle and soft. It is always mixed with a metal alloy for structure and durability. The most common sturdy and well made fine jewelry that can hold up and stay gold is 14k, 16k, 18k; this is the a happy well balanced medium.
5% 14kt gold bonded to 95% metal alloy (jewelry brass). Highest grade metal in the market and closest to solid without the price tag. Worry and hassle free. Hypoallergenic and water friendly to add.
92.5% silver mixed with 7.5% metal alloy. Hypoallergenic and water friendly to add.
Vermeil / Gold Plated over Sterling Silver
Gold that is bonded Sterling Silver. It can only be called Vermeil (Pronounced: ver-may) when it is .925 Sterling Silver base/core. Hypoallergenic and water friendly to add.
Less than 1% gold that is bonded to the metal alloy (jewelry brass). It's more of a finish to a metal, think of it a as garment dyed but in metal form.
Costume / Base Metals
Usually zinc or nickel based (sensitive people are prone this and these are two known metals people are allergic to) is treated with a flashed or dipped of gold, rose gold or finish to achieve the look desired. This is usually the jewelry you find in downtown fashion districts or imported fashion jewelry that are sold and produced dirt cheap. They are one time wear and toss type of jewelry.