Actually, diamond is inert in saltwater. Rather there is bacteria that can convert it to charcoal slowly, I don't know.
As for the gold. Most gold that you find is a alloy of copper, silver and gold. 16K gold has more than 1/3 non-gold metal, while 24K gold has a few percent of non-gold, mostly copper. White gold is not white from the Iridium coating. It is solid "white" thru out the entire metal. It is a mixture of gold, nickel, and others. Pure gold is very inert in seawater, and in fact, there is relatively high concentration of gold in natural seawater.