What metal is stronger: Platinum, Palladium, 14K White Gold or 18K White Gold?

Is it possible to have my old gold jewelry melted down so that I can keep the gold for when I want to make a new piece?

Mu husband bought a white gold chain from the antique shop ang there was a certificate of that saying it was a real gold after days or months of wearing it turns out black, now my question is it really a real gold or we've been scammed?