1870-S Proof

The 1870-S three-dollar gold piece is one of the great singular rarities of American numismatics, a coin known from a single surviving example and tied directly to the construction of the second San Francisco Mint. The piece was struck at the branch facility in 1870 to commemorate the laying of the cornerstone of the new Mint building, the granite structure that still stands at Fifth and Mission and is remembered as the Granite Lady. Period accounts indicate that a second specimen was placed inside the cornerstone itself, where it presumably remains, leaving the surviving piece as the only example available to study or display. The coin was prepared as a special-presentation strike rather than a regular proof issue, and no production figure was ever recorded because the engagement was ceremonial rather than commercial. The site mintage field reflects the surviving population of one, which is the only meaningful figure for a coin of this status. The Type 2 large DOLLARS reverse with the S mintmark is the format used, and the surviving example has been authenticated, cataloged, and preserved across more than a century of careful custody.

Pedigree functions as authentication in itself for a coin known from a single specimen. The unique example was acquired by Harry W. Bass in 1982 for $687,500, and following the formation of the Harry W. Bass Foundation it was transferred to permanent display at the ANA's Edward C. Rochette Money Museum, where it remains today as the centerpiece of one of the most important assemblages of United States gold ever formed. Any 1870 three-dollar gold piece bearing an S mintmark that does not trace through the Bass pedigree and is not authenticated by a major grading service must be presumed wrong without further investigation. The mintmark verification is the highest-stakes diagnostic, since added-S fakes built on a genuine 1870 Philadelphia host represent the most plausible counterfeit threat. Mirror proof fields are essential and are themselves a diagnostic, because no other San Francisco proof exists in the entire three-dollar series. Weight should fall at 5.015 grams within tolerance, diameter 20.5 millimeters, edge reeded, alignment ↑↓.

For collectors, the 1870-S three-dollar gold sits among the trophy coins of the entire federal series alongside the 1804 dollar and the 1913 Liberty Head nickel. The piece has never been sold publicly since the Bass acquisition, and a hypothetical appearance would almost certainly clear five million dollars. See the full Three-Dollar Gold series history.