Gold purity is broken up into karats, or carats outside North America. Solid gold is 24 karats and is usually mixed with alloys (other metals) for varieties in color, strength and cost. For jewelry it's combined with silver and copper, nickel has mostly been phased out because of it's toxicity. One karat (often stamped "K" or "KT") stands for 1/24th gold, so one can figure out the percentage of gold in a piece by dividing the karat by 24.
Different manufacturers use slightly different alloy mixtures, leading to tiny variations in the colors of the final product. While the amount of gold in the mixture is strictly regulated and the colors (yellow, rose, green, white) are relatively standardized, one might find a 14k yellow gold ring looks more silver-toned than another. The only difference in quality between the two rings would be if one had a higher quality alloy mixed in, like platinum or palladium.
The percentage of gold in the various karats:
24 karat = 99.99% gold or Pure gold
22 karat = 91.7 % gold
18 karat = 75.0 % gold
14 karat = 58.3 % gold
12 karat = 50.0 % gold
10 karat = 41.7 % gold
Pure gold, solid gold, Au on the Periodic Table of Elements. Known to be 99.99% pure. Since it's such a soft (malleable) material it is only used in jewelry applications in electroplating onto other metals. Being a noble metal it is extremely non-reactive and therefore does not tarnish. The largest quantities of pure gold are in coins and bars but is also used in electronic and medical devices because of it's unparalleled thermal and electrical conductivity rate. Its softness makes it extremely ductile and able to be hammered so thin it's almost transparent.
Pure gold is always the same color as there are no alloys. There is only 24K yellow gold, no rose, white, etc. It's deep, vibrant yellow sheen lasts indefinitely because of it's reflectivity properties.
Gold is a rare mineral, hence it's high value. It estimated that the total amount ever mined is only 125,000 tons.
22 parts gold mixed with 2 parts of alloy metals. It is the highest karat used to make jewelry and therefore the most expensive. It maintains the color and anti-corrosion properties of pure gold. The 8.33 percent of alloy metals mixed into the gold adds durability, but is still quite soft and therefore not the best choice for using with diamonds or heavily studded jewelry. It has a long history in Asia and the Middle East where jewelry is also used as in investment. India in particular has a long history of intricate gold work in 22K.
18 parts gold mixed with 6 parts of alloy metals. The percentage of alloys in 18K make it strong enough to use with diamonds and studded jewelry. Although it is a duller yellow than 22 and 24K, it is still a very vibrant yellow, making it a great balance between strength, color and resistance to tarnish. Though the highest karat version of rose gold, also known as crown gold, is 22 karat,18 karat is generally the highest karat you see rose, white, and green golds since it has enough alloy metals to see the colors.
Used world wide, especially in Asia, it is a great choice for engagement and wedding bands as it has a high gold content combined with the strength of 25% alloy metals. This makes it physically harder for jewelers to work with than higher karats, but still ductile enough to manipulate.
14 parts gold mixed with 10 parts of alloy metals. The increase in alloys makes it stronger than 18K, a lighter yellow, and a stronger rose, green and white color. 14K is where tarnishing starts to happen so pieces should be wrapped or in a air-tight bag when not in use. Popular in the UK and and the US and it is more affordable than 18K and higher.
10 parts gold mixed with 14 parts of alloy metals. Since this karat is more alloy than gold it is extremely hard, and tarnishes the most (not including 9K). It is the minimum standard to be called gold in the US. This is a good option for those with industrious jobs working with their hands. Commonly used in the UK and the US. It's is more of a challenge for a jeweler to work with not only because of it's physical strength and increased tarnishing, but when heating the high amounts of alloy metals rise to the surface and change the color of the piece. Jewelers have to take this into consideration as lots of metalsmithing techniques involve heat.
9 parts gold mixed with 15 parts of alloy metals. Countries outside the US will sometimes call this gold, but it is not commonly used. While uncommonly worked with today, it is popular in Ireland. It's pale yellow gives it a different look and is even stronger than 10K. It is used in the dental field.
Usually just the mark of fineness of precious metals. Most countries require that the type of metal of a piece be stamped with it's corresponding symbol that varies from country to country. A handful of countries require the hallmark to included the maker's mark of the company where the piece was made in addition to the stamp denoting the type of metal. Also, some countries will have their own stamp to be used with the maker's mark and quality stamps.
Not only do hallmarks vary from country to country, but from the time of purchase, a helpful clue when trying to identify the date of an antique piece of jewelry.
While there are regulations in most countries about hallmarking jewelry, some antique and handmade pieces are occasionally without. While one should always be cautious when buying jewelry, this does not necessarily mean that a piece is not what it is being sold as, just that you should inquire about the specifics and possibly do an assay. Additionally, small jewelry is often unable to be stamped. A piece needs a solid area of 1.25mm by 2.25mm or more to fit just the metal fineness stamp, so a delicate ring or earring stud just can't fit it, unfortunately.