5.90 ct CORKEITE – USA

Corkite is a gemstone that occupies a sophisticated space where phosphate and sulfate chemistries intersect. A rare member of the beudantite group, it is a lead iron phosphate sulfate that I have always admired for its earthy, "autumnal" palette. It typically presents in wonderful, rhombohedral crystals that range from a deep, olive green to a rich honey-yellow or dark, chocolate brown. While it is often found as druzy coatings, finding distinct, lustrous crystals is a true collector's victory. It possesses a vitreous to resinous luster that gives it a substantial, "heavy" presence despite its often diminutive size. It is a stone of structural complexity and quiet elegance, representing the fascinating "chemical recycling" that occurs in the weathered upper zones of lead-bearing ore deposits.

Historical Significance: Corkite is a vital species for mineralogists studying the oxidation of complex sulfide ores. Historically, it was often overlooked or confused with its arsenic-rich cousin, Beudantite, until its unique phosphate-dominant chemistry was confirmed. It represents a specific geochemical "moment" where lead and iron are stabilized by both phosphate and sulfate ions in a secondary environment. In the world of fine minerals, it is celebrated for its sharp, "pseudo-cubic" geometry, standing as a refined representative of the alumino-phosphate-sulfate (APS) supergroup. It serves as a beautiful reminder of the intricate balancing act the Earth performs to create rare, crystalline species from the breakdown of primary ores.

Discovery: The mineral was first described in 1869 and was named after its type-locality: County Cork, Ireland. Its discovery in the Glandore iron mine provided the first well-studied specimens of this complex lead-iron mineral. The naming honors the rugged, mineral-rich landscape of the Irish coast where it was first pulled from the earth, and it remains the definitive name for this phosphate-sulfate species worldwide.

Important Mines: While Ireland holds the historical title, some of the most spectacular, well-formed crystals have come from the Tsumeb Mine in Namibia and the Darmstadt region in Germany. I also have a high regard for the specimens found in the Broken Hill district of New South Wales, Australia. For the vault, I prioritize the German and Namibian material, as they often produce the most lustrous, "glassy" rhombohedrons that display the species' geometric perfection at its absolute best.

Description: Corkite is a lead iron phosphate sulfate hydroxide that crystallizes in the trigonal (rhombohedral) system. It sits at a 3.5 to 4.5 on the Mohs scale, making it a delicate mineral that should be handled with care and protected from harsh environments. It is characterized by its vitreous, resinous, or even sub-adamantine luster and its dark green to yellowish-brown color.

One of its most identifying traits is its habit—it almost always forms as small, sharp rhombohedrons that can appear nearly cubic to the untrained eye. Because it contains lead and iron, it is surprisingly heavy for its size, with a high density ($4.7$ to $4.8$ g/cm³). It has a high refractive index ($n \approx 1.93$ to $1.95$), which gives it a bright, "snappy" brilliance when light catches the crystal faces. In the field, it is often found alongside other secondary minerals like Pyromorphite or Limonite. When I select a piece for the collection, I look for "clean" crystal faces with a high-reflectivity luster, as these highlight the sophisticated, three-dimensional geometry that makes Corkite a standout in the world of rare phosphates.

Chemical Formula: PbFe³⁺₃(PO₄)(SO₄)(OH)₆