In another two decades rare commodities may become seriously scarce. According to Goldman Sachs, the world has about 20 years each of known minable reserves of gold GCJ5, -0.30% diamonds and zinc. Platinum PLJ5, +0.27% copper HGK5, -0.40% and nickel reserves only have about 40 years or less left. “The combination of very low concentrations of metals in the Earth’s crust, and very few high-quality deposits, means some things are truly scarce,” Eugene King, European metals and mining analyst at Goldman Sachs, wrote in a recent research note. “Gold has been used as a measure of wealth for more than 4,000 years, as the ... (full story)