White Gold
18k white gold is composed of 75% pure gold (which is yellow) and 25% white metals (silver, palladium, nickel, etc) and so 18k white gold does still have a slightly yellowish color compared to the rhodium plating.
Rhodium is used to give the white gold a shiner, whiter finish and to protect the gold from scratching as it is a soft metal. The rhodium plating lasts for several years, depending on the ring design, wear and thickness of the plating and can be re-dipped for a small fee at any local jeweler, when cleaned.
White gold
White gold's properties vary depending on the metals and proportions used. As a result, white gold alloys can be used for many different purposes; while a nickel alloy is hard and strong and therefore good for rings and pins, gold-palladium alloys are soft, pliable and good for white gold gemstone settings, sometimes with other metals like copper, silver, and platinum for weight and durability, although this often requires specialized goldsmiths.
The term white gold is used very loosely in the industry to describe karat gold alloys with a whitish hue. Many believe that the color of the *rhodium plating, which is seen on many commercial pieces, is actually the color of white gold. The term "white" covers a large spectrum of colors that borders or overlaps pale yellow, tinted brown, and even very pale rose. The jewelry industry often hides these off-white colors by rhodium plating.
A common white gold formulation consists of 90 wt.% gold and 10 wt.% nickel. Copper can be added to increase malleability.
The strength of gold-nickel-copper alloys is caused by formation of two phases, a gold-rich Au-Cu, and a nickel-rich Ni-Cu, and the resulting hardening of the material.
The alloys used in jewelry industry are gold-palladium-silver and gold-nickel-copper-zinc. Palladium and nickel act as primary bleaching agents for gold; zinc acts as a secondary bleaching agent to attenuate the color of copper.
[From http://en.wikipedia.org/wiki/Colored_gold]
*Rhodium
[Wikipedia] Rhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as the free metal, alloyed with similar metals, and never as a chemical compound. It is one of the rarest precious metals, and the most costly.
Rhodium is a so-called noble metal, resistant to corrosion, found in platinum- or nickel ores together with the other members of the platinum group metals. It was discovered in 1803 by William Hyde Wollaston in one such ore, and named for the rose color of one of its chlorine compounds, produced after it reacted with the powerful acid mixture aqua regia.
The element's major use (about 80% of world rhodium production) is as one of the catalysts in the three-way catalytic converters of automobiles. Because rhodium metal is inert against corrosion and most aggressive chemicals, and because of its rarity, rhodium is usually alloyed with platinum or palladium and applied in high-temperature and corrosion-resistive coatings. White gold is often plated with a thin rhodium layer to improve its optical impression while sterling silver is often rhodium plated for tarnish resistance.
