Platinum is even more rare and valuable than gold. The platinum family is composed of six elements- platinum, palladium, iridium, osmium, rhodium, and ruthenium. The six silvery white colored metals are generally found together in nature, with platinum and palladium the most abundant, and osmium, rhodium, and ruthenium, the rarest. Platinum is rarer and heavier than other precious metals and is the purest metal- often referred to as the "noblest." Because platinum is so pure it rarely causes allergic reactions. In addition platinum is somewhat stronger and retains its white color unlike white gold.
Unlike gold, platinum is not described in karats. In the US, abbreviations such as PT or Plat, indicate 95% platinum. Lower purities should be marked such as 585 plat or 900 plat. In Europe, the numerical markings 950 or PT950 indicate platinum.

Rhodium Plating
Rhodium, another member of the platinum family, is the brightest and most reflective of all the platinum metals. Because of these qualities it is frequently used to coat silver, gold, platinum, and palladium jewelry, as an electroplate finish. Rhodium is harder and whiter even than platinum, and highly resistant to corrosion.
Rhodium plating can eliminate allergic reaction caused by the alloys in karat gold and is also used to change the color of yellow gold to white for those people with an allergy who prefer white but can't afford platinum.