The content of gold is measured in karats, and not in carats, which is used to measure the weight of diamonds. As the K number increases, so does the gold content and price. Karats represent how many parts, by weight, pure gold is present in the 24 parts of the alloy. Below is the range of gold content:

10k = 10/24 = 41.7% pure gold
14k = 14/24 = 58.3% pure gold
18k = 18/24 = 75.0% pure gold
24k = 24/24 = 100% pure gold

Regarding the decision of which karat to select, it is a matter of personal taste. 14k gold is stronger than 18k, however 18k has a higher gold content and a richer yellow color.

Gold is extremely rare. It is estimated that all the gold that can be mined would fit under the Eiffel Tower in Paris. Additionally, it takes a few tons of ore to produce only one ounce of gold. Gold has a beautiful, warm golden color that can be mixed with many other metals to produce an array of other beautiful colors. As an example, mixing copper with gold creates rose gold, mixing gold with silver creates green gold, and mixing gold with palladium creates white gold. Gold is exceptionally durable. The sheer number of ancient gold artifacts and jewelry that we have today attest to the strength and permanence of its beauty. Pure gold is quite soft, yet when combined with other metals, it takes on indestructible characteristics. Gold is pure, and because of this, it is resistant to the air, heat, moisture, and therefore does not tarnish. It is also very malleable and easy to work with as it can be melted into any kind of design. These combined characteristics have made gold a tremendously valuable commodity.