The Logan Sapphire is a 423.0-carat (some sources cite 422.99 carats) cushion-cut blue sapphire of Sri Lankan origin, mounted as a brooch with a surround of diamonds and held in the National Gem Collection of the Smithsonian National Museum of Natural History in Washington, D.C. It is among the largest faceted blue sapphires in the world and has been on continuous public display in the museum's Hall of Geology, Gems and Minerals since the 1960s.

Origin and history

The stone was mined in Sri Lanka, then Ceylon, at an unrecorded date in the late nineteenth or early twentieth century. The exact mine locality has not been documented in published sources, although the stone's gemmological characteristics are consistent with the Ratnapura district, the principal sapphire-producing region of the country. Sri Lankan sapphire from Ratnapura is typically alluvial in origin, recovered from gem gravels worked by hand.

The brooch in which the stone is now set was made by Cartier and was given to Mrs Polly Logan (Mrs John A. Logan) by her husband. Mrs Logan donated the brooch to the Smithsonian in 1960, where it joined the Hope Diamond and the Star of Asia sapphire as principal exhibits in the gem collection. The stone is named for the donor.

Gemmological characteristics

The Logan Sapphire is a deep, slightly violetish blue, displaying the characteristic Sri Lankan body colour rather than the more saturated, slightly inky blue of fine Kashmir or Burmese material. The stone is faceted as a cushion mixed cut with a brilliant crown and a step-cut pavilion, an arrangement standard for large sapphires of the late nineteenth and early twentieth centuries. Sri Lankan sapphires of this size and clarity are exceptionally rare; the combination of the stone's size, its clarity grade and its strong colour saturation make it one of the more important named stones in any institutional collection.

The stone has not been publicly reported as heated or unheated. The Smithsonian's published material describes its origin and provenance but does not address treatment history; given the era of its discovery, it likely predates the modern heat-treatment industry and is presumably untreated, although no laboratory report has been made public.

Display and significance

The brooch is mounted in platinum with twenty round brilliant-cut diamonds totalling roughly sixteen carats arranged around the central sapphire. The setting is consistent with high-quality Cartier work of the early twentieth century. The brooch is kept under acid-free, controlled-atmosphere display conditions in the Smithsonian's Janet Annenberg Hooker Hall and is one of the most photographed objects in the National Gem Collection.

Among named sapphires, the Logan ranks behind the 563.35-carat Star of India (a star sapphire at the American Museum of Natural History in New York) and the 392.52-carat Black Star of Queensland (a star sapphire in private hands) in carat weight, but ahead of most other documented faceted blue sapphires of public record.