What tarnishes white gold and diamonds?

If your particular white gold is a mixture of Nickel and gold vs gold/ palladium alloy, you might be able to corrode it by using something that will go after nickel. The Au/Pd alloy is considerablly more corrosion resistant. Anytime you have a mixture of Cl and NO3 compounds, you may produce a bit of aqua regia. Aqua regia will disolve white gold. In other words, bleach(cl) in the presence of fertilizer might do it.