The karatage shows how much of the metal is pure gold. 24k is 99.9%-100% gold, while 18k is 75% gold. Pure gold is too soft for everyday wear, so gold jewelry often also contains other metals, such as silver, copper, nickel or zinc, to make the piece of jewelry more durable.
edit Difference in Appearance
Since 18k gold is mixed with various alloys like silver, copper, zinc and nickel, the yellow color of 24k gold is more rich and vivid than in 18k gold.
18k gold is the most common type of gold used in Europe and Asia.It is extensively used for making fine jewelry. It is durable , but since it is less resistant than 14k gold, it shows marks after some rough use.
24k gold is considered too soft for fine jewelry, it is most often used for investment purposes. Gold coins and bars are minted in fine gold with purities of 99.9% – 99.99%.
edit Other Terms
As 24k gold is 99.9-100% gold, it is also be referred to as pure gold or fine gold. 18k gold can be referred to as 750 gold.
edit Video explaining the differences
This is a good informational video that talks about the key differences between various types of gold:
The price of gold jewelry varies by design, craftsmanship and the alloys used, but as 24k contains more gold than 18k, it is usually more expensive.
The value of 18k gold can be calculated by multiplying its pennyweight by .750, dividing the current spot gold price by the number of pennyweights per ounce, and then multiplying the two values together.
The value of 24k gold is calculated by dividing the current spot gold price by the number of pennyweights per ounce, and then multiplying this value to the pennyweight of the product.
While gold has historically been cheaper than platinum, in late 2011, prices for pure gold rose above the price for pure platinum.
Prices of some gold jewelry are available on Amazon.com: