A karat (K) is the unit of measurement for the proportion of gold in a metal alloy. Since 24 karat gold is 99.9 percent gold, its composition is the closest to pure gold. Twenty-four karat gold is soft but durable; it will never tarnish and is hypoallergenic. Eighteen karat gold or 75 percent gold has been combined with other metals, making it stronger than 24 karat gold. However, this can cause the alloy to tarnish and cause allergic reactions on sensitive skin. Eighteen karat gold is almost always worth less than 24 karat gold, as gold prices are based on the purity of the metal.

Yellow gold is usually made from a higher karat and contains only gold, while white gold contains platinum, palladium or silver, and is usually rhodium plated. Rose gold has been combined with copper, giving it a rosy color. This type of gold is usually created from 18K gold. As a general rule, yellow gold costs more than white gold, and white gold costs more than rose gold.

For investors in gold bullion, a refined gold which has been converted from scrap into an easily tradable form, the gold is not purchased for its physical durability but for its economic stability, as gold tends to retain its value.

Though you will receive more money for 24 karat gold scrap than 18 karat gold scrap, you will not receive the going rate, because those who buy scrap subtract the cost of melting the gold down from the amount you receive.

Gold as a dental crowns or fillings is often of the 18 karat variety. Though gold fillings tend to cost more, they also last two to four times longer than porcelain and amalgam fillings.

Eighteen karat gold is almost always worth less than 24 karat gold, as gold prices are based on the purity of the metal, and 24 karat gold is more pure than 18 karat gold. This holds true when melting gold down for scrap or turning gold into bullion as a financial security blanket in tough economic times.

There are times when 24 karat gold is worth less than 18 karat gold. When the latter is mixed with platinum, it is worth more, because platinum is four times as costly. Eighteen karat gold used in art and jewelry can be worth more than 24 karat gold when a price is put not just on the karats used in the design, but on beauty of the piece, as well.

You can find the current price of gold by calling your local jewelry store, coin shop or pawn shop, as these store owners will know gold’s current rate per ounce or gram. Separate your gold by karats. Divide the karat by the purest known karat, or 24. Take that number and multiply it by today’s gold price per gram. For example, if you have an 18 karat ring that weighs 1 gram, and the price per gram is $26, then you take 18 divided by 24 and multiply your answer by 26. This gives you the price per gram of 18 karat gold, which is $19.50.