1.75 ct TARBUTTITE – SOUTH AFRICA

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Tarbuttite is a gemstone that represents the perfect marriage of secondary zinc chemistry and high-level symmetry. A rare zinc phosphate, it is a mineral that I have always admired for its sharp, “sculptural” presence. It typically forms in wonderful, triclinic crystals that are often sheaves or bundles of distinct, bladed forms. In its purest state, it is colorless or a pale, icy white, but the presence of copper can sometimes tint it with a ghost of green. To look at a well-formed cluster of Tarbuttite is to see the Earth’s ability to create clean, geometric perfection in the quiet, oxidized pockets of a zinc mine. It is a sophisticated stone for the collector who prizes clarity and rare phosphate species, offering a bright, vitreous brilliance that stands out in any display of rare gems.

The Heritage & Discovery

Historical Significance: Tarbuttite is a classic indicator of the oxidation of zinc ores in a phosphate-rich environment. Historically, it was a significant discovery within the “broken hill” type deposits, where it was often found alongside other rare species like Hopeite. It represents a specific geochemical moment where zinc, phosphorus, and oxygen align under low-temperature, surface-near conditions. In the world of fine minerals, it is celebrated for its complex triclinic forms and its historical connection to the Great African mining boom of the early 20th century. It stands as a refined representative of the phosphate family, proving that even “industrial” metals like zinc can produce treasures of immense aesthetic value.

Discovery: The mineral was first described in 1907 and was named in honor of Percy Coventry Tarbutt, a director of the Broken Hill Exploration Company. He was instrumental in the development of the mining operations at the type-locality in what was then Northern Rhodesia. His dedication to exploring the mineral wealth of the region led to the identification of this unique phosphate, ensuring his name remains a permanent part of the mineralogical record.

Important Mines: The legendary and undisputed type-locality for Tarbuttite is the Broken Hill Mine (Kabwe Mine) in Kabwe, Zambia. This site has produced the world’s finest and most iconic specimens, featuring the sharp, bladed crystals that define the species. I also have a high regard for material found in the Reaphook Hill area of South Australia. For the vault, I prioritize the Zambian material, as its historical provenance and the sheer quality of its crystalline sheaves are the global benchmark for the species.

Mineralogical Profile

Description: Tarbuttite is a zinc phosphate hydroxide that crystallizes in the triclinic system. It sits at a 3.5 to 4 on the Mohs scale, making it a relatively soft and delicate mineral that is best suited for protected display or specialized collections. It is characterized by its vitreous luster and its typically colorless, white, or pale yellow to green appearance.

One of its most identifying traits is its habit—it almost always forms as small, bladed crystals that are often grouped into “sheaves” or radiating clusters. It features a perfect cleavage in one direction, which gives the crystal faces a particularly bright, reflective quality. It has a high refractive index ($n \approx 1.66$ to $1.71$), which provides a surprising amount of “snap” and brilliance for a colorless stone. When I select a piece for the collection, I look for specimens with the “classic” Zambian sheaves, as the interplay of light across the many small, bladed faces creates a shimmering effect that is unique to this rare phosphate.

Chemical Formula: Zn₂(PO₄)(OH)