The difference is the amount of pure gold versus alloy. For gold to be durable enough for most jewelry it needs to be mixed with other metals. For example, pure gold is listed as 99.99% pure, or 24kt. 18kt gold is 75% gold and 25% alloy, while 14kt. is 58% gold and 42% alloy. 10kt. contains the least amount of gold at 42% to 58% alloy. The alloys determine the color of the gold. For example, if more copper is used the gold becomes pink or rose color. Nickel and other metals from the platinum group help to make white gold. Which is best? It's really personal preference. However, don't let someone tell you that you need a 10kt. piece of jewelry because it's harder. The reason 10kt is popular is because of its affordability. It has less gold, so it's less expensive. 14kt and 18kt have a very strong place in the jewelry industry and many fine pieces are made in no less than 18kt.