You can find a starter list here.
Gold, silver, platinum, palladium, are ill precious metals, but there are also a lot of other metals that are valuable, such as nickel, aluminum, copper, magnesium. There are a whole host of commodities that have value.
When Nat’l Geographic (or anyone) does an exercise like that, they take estimates of the known amounts and multiply it by the current value as reflected on commodity exchanges. Then they add it up, et voila! A sum of the current value of the untapped resources.