According to Gillett's Jewellers, a primary difference between white gold and platinum is that white gold is typically an alloy of gold and some white metals, while platinum is a white metal, so platinum is used in jewelry in a much purer form. Platinum is also very dense, which makes it heavier than white gold.
The gold content of white gold in jewelry depends on its karat. For example, an 18-karat white gold ring is made by mixing 3 parts white gold to 1 part of some other metal, such as silver or palladium. A platinum ring is always approximately 95 percent pure platinum.
After white gold rings are made, they are coated with rhodium. White gold is actually light grey in color, so rhodium is added to make the gold allow appear more white in color. Although rhodium plating is very hard, it does wear away over time, so it must be re-applied after approximately 12 to 18 months. Unlike white gold, platinum is extremely hard and white, so it does not require rhodium plating. Platinum is also more expensive than white gold. As of 2014, a platinum ring of the same size is approximately twice the price of a white gold ring.