Everyone is familiar with gold and silver coins and bullion. What other metals are available as bullion or coins, or other tradable form?
One way to measure liquidity would be to examine the difference in the "Bid" and "Ask" prices of the different metals.
Gold has a difference of $1 on a price of $1348.40 (0.07 %)
Silver has a difference of $0.05 on a price of $23.25 (0.22 %)
Platinum has a difference of $10 on a price of $1705.50 (0.59 %)
Palladium has a difference of $5 on a price of $589.20 (0.85 %)
Gold has a very small spread for its price. To have such a small spread it must be traded a lot to determine the market price for both buyers and sellers. It seems to make sense that gold and silver have the lowest spread (and thus more liquid) since they are more well known.
There is a copper bullion coin. It's called the nickel. :)
Platinum is one of the metals available as bulion coin;
If you are also considering ETF trading then, Platinum and Palladium are also available.
As bullion coin it is apparently gold, silver, platinum and palladium:
I don't know which are readily traded, but I'd assume it is the first three in that list.
Wikipedia also has a list of previous metals, on the bottom of this page. That doesn't exactly answer the question of which have a relatively liquid market.