Platinum
Platinum is a popular white metal. This is due in part to its ability to remain white, and so does not need to be rhodium plated. Platinum is denser than white gold, so it will be heavier & considered more rare than a white gold ring.
Palladium
Palladium is a naturally occurring metal that is 95% pure. It is hypoallergenic, therefore will not irritate sensitive skin. It doesn't need to be treated with rhodium, as white gold does, because it is a naturally white metal. For those not comfortable with the heavier weight of a platinum ring, palladium is the better choice.
White Gold
White gold is mix of natural gold and other white alloys including silver and palladium. When white gold rings are new, they are coated with another very hard white metal called Rhodium to make the white gold look whiter. To maintain a white gold ring looking its best it should be re- plated approximately each 12 to 18 months.
Yellow Gold
Yellow gold is made by mixing pure gold with alloys including copper and zinc. The percentage of gold within the combination determines the carat of the gold i.e. 18ct yellow gold has 75% pure gold. For everyday wear 18ct is the best choice for engagement rings and weddings bands as it is stronger and continues to look well over time.