Carat: The Measure of Gold's Purity
What is a Carat? Beyond the Diamond
Many people first hear the word "carat" in the context of diamonds, where it is a unit of weight (equal to 200 milligrams). However, when applied to gold, "carat" (or "karat" in American English) has a completely different meaning. It is not a measure of weight but a measure of purity.
Pure gold, known as 24-carat gold, is too soft for most practical uses. If you made a ring from it, it would quickly bend, scratch, and lose its shape. To make gold stronger and more durable, it is mixed, or alloyed, with other metals like silver, copper, nickel, or zinc. The carat system tells us exactly how much of that mixture is actually pure gold.
$ \text{Percentage of Pure Gold} = \frac{k}{24} \times 100\% $
For example, for 18-carat gold: $ \frac{18}{24} \times 100\% = 75\% $. This means 18k gold is 75% pure gold and 25% other metals.
The Carat Scale: From 24k to 10k
The carat scale divides pure gold into 24 parts. The carat number tells you how many of those 24 parts are pure gold. Here is a breakdown of the most common carat values used in jewelry worldwide.
| Carat (k) | Parts Gold (out of 24) | Percentage Purity (%) | Common Uses & Properties |
|---|---|---|---|
| 24k | 24/24 | 99.9% (or higher) | Investment bullion, coins. Very soft, bright yellow, easily scratched. |
| 22k | 22/24 | 91.7% | Traditional jewelry in many cultures. More durable than 24k but still somewhat soft. |
| 18k | 18/24 | 75% | High-quality jewelry. Excellent balance of purity, color, and strength. |
| 14k | 14/24 | 58.3% | The most popular choice for everyday jewelry like rings and bracelets. Very durable. |
| 10k | 10/24 | 41.7% | Common, affordable, and extremely durable. The minimum legal standard to be called "gold" in the US and Canada. |
How Alloying Creates Different Colors of Gold
Mixing gold with other metals doesn't just make it stronger; it also changes its color. This is how we get the beautiful variety of white, rose, and even green gold jewelry. The alloy metals act like pigments.
- Yellow Gold: Alloyed with silver and copper.
- White Gold: Alloyed with white metals like nickel, palladium, or manganese. It is often plated with a very thin layer of rhodium to enhance its bright white appearance.
- Rose Gold (or Pink Gold): Alloyed with a higher proportion of copper. The more copper added, the redder the color becomes.
- Green Gold: A less common type, alloyed primarily with silver, which gives it a subtle greenish tint.
It's important to remember that a piece of 18k rose gold and an 18k white gold ring contain the same amount of pure gold (75%). The different colors come from the 25% of other metals.
Calculating Gold Content in Real-World Objects
Let's apply the carat knowledge to a practical example. Imagine you have an old gold necklace. It weighs 40 grams and is stamped "18k". How much pure gold do you actually have?
Step 1: Find the purity percentage. 18k is $ \frac{18}{24} = 0.75 $, or 75% pure gold.
Step 2: Calculate the weight of the pure gold.
$ \text{Weight of Pure Gold} = \text{Total Weight} \times \text{Purity Percentage} $
$ \text{Weight of Pure Gold} = 40 \text{ g} \times 0.75 = 30 \text{ g} $
So, the 40-gram necklace contains 30 grams of pure gold. If the market price of gold is $60 per gram, the intrinsic value of the gold in the necklace would be 30 g × $60/g = $1800. (Note: A jeweler would pay slightly less than this market value).
Hallmarks: The Official Stamp of Purity
How can you trust that a piece of jewelry is really the carat it claims to be? This is where hallmarks come in. A hallmark is an official mark stamped on precious metals by an assay office[1] after formal testing. These tiny stamps guarantee the purity of the metal.
Common hallmarks you might find on gold include:
750 - This means 750 parts per thousand pure gold, which is the same as 18k (18/24 = 0.75 = 750/1000).
585 - This means 585 parts per thousand, which is 14k (14/24 ≈ 0.5833).
417 - This means 417 parts per thousand, which is 10k (10/24 ≈ 0.4167).
You will also often see the carat number itself stamped, like "18k" or "14k".
Common Mistakes and Important Questions
A: Not necessarily. While a higher carat means more pure gold (and thus a higher intrinsic value), it also means a softer, less durable metal. For an investment bar, 24k is best. For an engagement ring that will be worn every day, 14k or 18k is a much better choice as it will resist scratching and bending.
A: This is a common point of confusion. Carat (ct) is used almost exclusively as a unit of weight for gemstones (like diamonds). Karat (k) is the American English spelling for the measure of gold purity. Outside of North America, "carat" is used for both, so context is key! If it's about a diamond, it's weight. If it's about gold, it's purity.
A: It can, but it is not practical for rings or bracelets that face a lot of wear. It is, however, popular for certain types of necklaces, earrings, and especially in many Asian and Middle Eastern cultures for bangles and traditional jewelry where the deep yellow color is highly valued and the pieces are often designed with the softness in mind.
Footnote
[1] Assay Office: An official body that tests and verifies the purity and quality of precious metals like gold and silver.