Platinum, Yellow Gold, White Gold, Rose Gold, Crown Gold, Purple Gold and Blue Gold

One of the managers asked me what “White Gold” is? Is that Platinum? I said white Gold is “Gold” mixed with certain percentage of other elements like Zinc, copper. Platinum is another white grey colour element but totally another different element.
Interpretation of “White Gold” is “Platinum”… this is common wrong perception of most of the people.
Platinum
Platinum is a white metal, but unlike gold it is used in jewelry in almost its pure form (approximately 95% pure). Platinum is extremely long wearing and is very white, so it does not need to be Rhodium plated like white gold. Platinum is very expensive, dense (heavy), so a platinum ring will feel heavier than an 18kt gold ring.
Yellow Gold
Pure Gold 24kt is comparatively too soft and is not advised to make watches.
Gold has its own colour that no other metal can replace. The Chinese temple has been painted with gold colour as the paint has blended with powder of gold.
White Gold
White gold is an alloy of gold and some white metals such as silver, Zinc, Copper and palladium. White gold can be 18kt, 14kt, 9kt or any karat. For example, 18kt yellow gold is made by mixing 75% gold (750 parts per thousand) with 25% (250 parts per thousand) other metals such as copper and zinc. 18kt white gold is made by mixing 75% gold with 25% other metals such as silver and palladium. So the amount of gold is the same but the alloy is different. Traditionally nickel was used in white gold, however, nickel is no longer used in most white gold made today as nickel can cause reactions with some people. We do not use nickel in our white gold. When white gold rings are new they are coated with another white metal called Rhodium. Rhodium is a metal very similar to platinum and Rhodium shares many of the properties of platinum including its white color. The rhodium plating is used to make the white gold look more white. The natural color of white gold is actually a light grey color. The Rhodium is very white and very hard, but it does wear away eventually. To keep a white gold ring looking its best it should be re-rhodium plated approximately each 12  months.
Rose Gold
Rose gold is a gold and copper alloy widely used for specialized jewelry. It is also known as pink gold and red gold. As it was popular in Russia at the beginning of the 19th century, it is also known as Russian gold, although this term has become somewhat rare.Although the names are often used interchangeably, the difference between red, rose, and pink gold is the copper content – the higher the copper content, the stronger the red coloration. A common alloy for rose gold is 75% gold and 25% copper by mass (18 karat). Since rose gold is an alloy, there is no such thing as "pure rose gold".
A common formulation for red gold is 50% gold and 50% copper. Up to 15% zinc can be added to copper-rich alloys to change their color to reddish yellow or dark yellow.
Crown gold
The highest karat version of rose gold is also known as crown gold, which is 22 karat. 18kt red gold may be made of 25% copper and 75% gold. For 18 karat rose gold, typically about 4% silver is added to 75% gold and 21% copper to give a rose color. 14 kt red gold is often found in the Middle East and contains 41.67% copper.
Purple gold
Purple gold is also called amethyst gold and violet gold is an alloy of gold and aluminum rich in gold-aluminium intermetallic (AuAl2). Gold content in AuAl2 is around 79% and can therefore be referred to as 18 karat gold. Purple gold is more brittle than other gold alloys, as it is an intermetallic compound instead of a malleable alloy, and a sharp blow may cause it to shatter. It is therefore usually machined and faceted to be used as a "gem" in conventional jewelry rather than by itself. In Chang Yi airport of Singapore, there is a gold smith selling Purple Gold, It is purple and shinning, quite attractive for jeweler, but I guess you can not tell it is gold. At lower content of gold, the material is composed of the intermetallic and an aluminium-rich solid solution phase. At higher content of gold, the gold-richer intermetallic AuAl forms; the purple color is preserved to about 15% of aluminium. At 88% of gold the material is composed of AuAl and changes color. The actual composition of AuAl2 is closer to Al11Au6 as the sub-lattice is incompletely occupied.
Blue Gold
Blue gold is an alloy of gold and indium. It contains 46% gold (about 12 ct) and 54% indium, forming an intermetallic compound AuIn2, with a clear blue color. With gallium, gold forms an intermetallic AuGa2 (58.5% Au, 14ct) which has slight bluish hue. The melting point of AuIn2 is 541 °C, for AuGa2 it is 492 °C. AuIn2 is less brittle than AuGa2, which itself is less brittle than AuAl2.
All the AuX2 intermetallics have crystal structure of CaF2 and therefore are brittle. A small amount of palladium, copper or silver can be added to achieve a less brittle microstructure. The intermetallic compounds tend to have poor corrosion resistance. The less noble elements are leached to the environment, and a gold-rich surface layer is formed. Direct contact of blue and purple gold elements with skin should be avoided as exposition to sweat may result in metal leaching and discoloration of the metal surface.
A surface plating of blue gold on karat gold or sterling silver can be achieved by a gold plating of the surface, followed by indium plating, with layer thickness matching the 1:2 atomic ratio. A heat treatment then causes inter-diffusion of the metals and formation of the required intermetallic compound.
HK Snob