Toning is a natural oxidation process than can produce multiple colors on a coin’s surfaces over time. The extent of the toning and the depth of color depends on the metallic composition of the coin and environmental factors under which a coin is stored.
Aesthetically pleasing natural toning can add value to a coin, while distracting natural or artificially created toning can decrease a coin’s value.
Common date, rarer subtype
The 1879-S Morgan dollar has a reported mintage of 9.11 million coins, which makes it common as a date.
The San Francisco Mint struck two significant subtypes or versions of 1879-S Morgan silver dollars, the result of continued experiments at the Philadelphia Mint to tweak the coin’s designs to improve strikeability in its second year of production. One of those variants of the coin is scarcer than the other major variant, though that scarcity was not a factor in the auction.
Some of the 1879-S coins bear what is identified as the Second Reverse, or Reverse of 1878, while others bear the Third Reverse, referred to as the Reverse of 1879.
Silver Dollars & Trade Dollars of the United States: A Complete Encyclopedia by Q. David Bowers, with Mark Borckardt describes the Reverse of 1878 as displaying a parallel top arrow feather and concave breast on the eagle. On the Reverse of 1879 version, the top arrow feather is slanting and the eagle’s breast is described as convex.
The authors estimate that 500,000 of the mintage are of the rarer Reverse of 1878 coins, while the remainder of is of the more common Reverse of 1879 subtype.
The more common subtype or variant is listed in Coin World’s Coin Values in Mint State 65 at $150; in the PCGS Price Guide at $165; and in Coin Dealer Newsletter at $138 Bid, $150 Ask.
The rarer 1879-S Morgan, Reverse of 1878 dollar is listed in MS-65 at $6,000 in Coin Values; at $5,000 in the PCGS Price Guide; and at $4,750 Bid, $5,200 Ask, in Coin Dealer Newsletter.