White Gold is a metal alloy, consisting of a mixture of gold and one or more white metals-- usually nickel, manganese or palladium. In the Chronicles of Thomas Covenant the Unbeliever, white gold can be used to channel (or more accurately unleash) Wild Magic.
During the First Chronicles and Second Chronicles, the Land's only known object made of white gold is Thomas Covenant's wedding ring. At the end of the Second Chronicles, it passed from Covenant to Linden Avery. During the Last Chronicles, Linden Avery still has it and uses it several times, arguably with more skill and precision than Covenant did.
However, Joan Covenant was also translated to the Land in the Last Chronicles, and she had her wedding ring with her (which Thomas Covenant later reclaimed). Her unbalanced mental state led to the creature of ''caesures'', which were dangerous flaws in time; some of these functioned as portals, allowing people to pass into other eras, while others were simply destructive and ravaging occurrences.
One stricture of white gold appears to be that it must be given willingly from a rightful user to another. Lord Foul was not able to take the white gold from Covenant by force; he had to be given the ring by Covenant. Likewise, Linden Avery would not have been able to use the ring if it hadn't come to her by choice.
White gold's power lies in working around the limitations of the Law that binds the Land. Its power surpasses any creation of the Lords, old or new, including the Staff of Law (Covenant's ring destroyed the original Staff when it opposed and sought to tame the wild magic). Although even the new Lords know something of wild magic and white gold, it appears that even High Lord Kevin understood very little about its nature. It is telling that the new Lords are able to give Covenant very little useful information about how his power may be wielded, forcing him to learn most of what he knows through trial and error.