White Gold
Gold alloyed with white metals and then usually rhodium plated. The white you see is a coating, not the metal.
White gold is yellow gold mixed with palladium, nickel, silver or zinc to lighten it. On its own the result is a faint, slightly warm grey rather than a true white.
Nearly all of it is therefore rhodium plated, which delivers the bright mirror white people actually buy.
That plating wears. On a ring it can start showing a warm tint at the edges within a year or two.
What you will hear
“White gold does not tarnish or change colour.”
In fact: The rhodium plating hides the alloy's natural warmth. As the plating wears through, the yellow underneath begins to show, and the piece needs re-plating.
When you are buying
Ask what re-plating costs before you buy, and treat it as a maintenance item like restringing a pearl strand. If you want a permanent white metal instead, look at platinum.
Rhodium Plating
A thin layer of rhodium over silver or white gold, giving a bright white finish that resists tarnish.
Platinum
A naturally white precious metal, denser and rarer than gold. Never needs plating, and does not fade.
Yellow Gold
Gold alloyed with silver and copper in balance, keeping its natural warm colour. The traditional Indian choice.
18 Karat Gold
75% gold. Harder and more durable than 22K, which is why stone-set and Western-style jewellery uses it.
Hypoallergenic
Metal unlikely to cause a skin reaction. In practice it means nickel-free, since nickel causes most jewellery allergies.