Platinum
Platinum, with the atomic symbol Pt, was not always a popular metal for jewelry and other trinkets. Due to the fact that it is rather difficult to work with and is slightly higher maintenance than other metals, gold was usually chosen over platinum. Extremely popular during the late 19th century up until WWII, platinum was the foremost choice for important jewelry. Platinum has come back into fashion in recent years and is the first pick for diamond engagement rings because it brings out a diamond's brilliance better than any other metal.
Platinum has a unique luster, is extraordinarily strong and weighs twice as much as gold. It is for this reason that it is chosen for jewelry (it is extremely durable) and it is also up to four times more expensive than gold, therefore making it more of a luxury item. The reason why it is pricier than gold or silver, apart from its weight, is due to its rarity. Annual production of platinum worldwide is only about 160 tons, compared to gold production of 1500 tons. It is also much more difficult to mine and refine platinum ore.
Just as with silver and gold, platinum is also alloyed with other metals, usually copper or titanium, but since platinum is so resilient, it is up to 95% pure. Marks such as 900Pt, 950Pt or Plat. on jewelry indicate the metal and its purity.