Karat is define as a unit of fineness for gold equal to 1/24 part of pure gold in an alloy. It means how pure a gold is. Alloy means the degree of mixture with base metal, gold being the base metal. A 24 Karat gold is equal to 100% pure gold. But we never say 100%, because no one has ever refine them to that point. The highest purity achieved is 99.99%, so a 24 Karat gold is 99.99% pure. Most of Hong Kong jewelry have a 9999 mark on them. And all branded gold bar too.
A 22 K gold from gold mine will have 91.6% pure gold while the remaining 8.4% will consist of any metal- silver, copper, iron, aluminum, etc. You just divide 22 K by 24 K to get an answer of 91.6%. Singapore jewelry always mark them with a 916. If I ask you how much pure gold content is in a 10 K gold. The answer is 10/24=41.6% pure. In jewelry the alloy added to make them to lower karat are always a mixture of silver and copper. No other metal can be mix with gold because it will later become blacked or rusted depending on what you put into it. 585 is 14 K.
We have 1 K, 2 K, 8 K, 23 K or 23.5 K of jewelry. Any karat is possible, it only depend on the jeweler who mix and melt them.
White gold here in the Philippines is not Platinum, it is only a mixture of gold and nickel (a malleable, silvery metal). We do have Platinum here in the Philippines but seldom seen them make into jewelry. The price for 24K Platinum is higher than the 24K yellow gold.
Source - WEB