2006 was the first time that the US MInt ever made a Burnished Gold Coin. The US Mint makes 3 different surfaces for Golf coins. They make a Mint State BU Gold Eagle, Proof Gold Eagle and Burnished Gold Eagle. All 3 coins are 100% 999 fine Gold. The burnished just has more of a satan finished.and it is defined on the label as SP70 ( Specimen Proof) The 2006 4 pc Gold Eagle Burnished Set is extremely rare. It’s the first year of issue and the mintage on the 4 pc set is lowest in the 16 years they have made the coins. If you search for the 2006 4pc Gold Eagle Burnished Set SP70 PCGS it is impossible to find. But find a 2006 4pc set Burnished