This word has two distinct meanings in the world of numismatics, so you have to consider the context in order to discern the correct meaning. The word "burnished" can refer to specially prepared planchets (usually 18th century) that were used for specimen coins or other special coins of the era. These planchets were burnished at the Mint prior to the striking of the coin. As a second meaning, "burnished" can refer to any coin that was abrasively cleaned after it left the Mint, and the word is often used as a synonym for "whizzed" (the worst kind of cleaning, where the metal is actually moved around).