4.35 ct COLEMANITE – BRAZIL

Colemanite

Colemanite is a gemstone that represents the “pure,” crystalline heart of the borate world. A hydrated calcium borate, it is a mineral I have always admired for its incredible, “water-clear” transparency and its sharp, architectural crystal forms. It typically presents as colorless to snow-white, though it can occasionally show a delicate “honey-yellow” or “smoky” tint. For the specialist, Colemanite is a stone of the ancient desert lakebeds, representing the slow evaporation of boron-rich waters in arid environments. It is a mineral of exceptional brilliance; its vitreous to pearly luster and high-gloss facets make it look like a fragment of frozen desert light. It is a prize for the vault, where its geometric precision and “icy” depth can be fully appreciated.

The Heritage & Discovery

Historical Significance: Colemanite is a vital species for understanding the concentration of boron in “evaporite” deposits. Historically, it was the primary industrial ore of boron before the discovery of Kernite, fueling the glass and ceramic industries of the early 20th century. In the world of fine minerals, it is celebrated for its monoclinic symmetry and its ability to form large, aesthetic crystal groups. It stands as a symbol of clarity and endurance, bridging the gap between raw industrial minerals and the elite world of rare, faceted collector gems. It serves as a reminder that even in the harshest, driest environments on Earth, nature can crystallize structures of immense purity and grace.

Discovery: The mineral was first described in 1884 and was named in honor of William Tell Coleman (1824–1893), a legendary merchant and the owner of the mine where it was first identified. The type-locality is Death Valley, California, USA. Naming this resilient and “bright” mineral after a man who was instrumental in the development of the American West was a fitting tribute to his pioneering spirit. Since its formal identification, it has become the definitive representative of the borate family for collectors of North American minerals.

Important Mines: The undisputed world capital for world-class Colemanite is the Boraxo Mine and the Ryan Mine in Death Valley, California, USA. These sites have produced the most well-defined and large-scale crystals ever found. I also have a high regard for the spectacular material found in the Bigadiç Mine in Balıkesir Province, Turkey, and the “gemmy” specimens from Kazakhstan. For the vault, I prioritize the California material for its unmatched historical pedigree and the Turkish pieces for their exceptional crystal size and “water-clear” transparency.

Mineralogical Profile

Description: Colemanite is a hydrated calcium borate that crystallizes in the monoclinic system. It sits at a 4.5 on the Mohs scale, making it a relatively soft mineral that should be protected from physical shock and harsh chemicals. It is characterized by its vitreous luster on crystal faces and a distinct pearly luster on its cleavage planes.

One of its most identifying traits is its pyroelectric and piezoelectric properties—meaning it can generate an electric charge in response to changes in temperature or mechanical stress. It has a moderate refractive index ($n \approx 1.58$ to $1.61$), which provides a rich, “inner” glow when light passes through the crystal. Because it is a hydrated borate, it is sensitive to heat and should be stored in a stable environment. When I select a piece for the collection, I look for “sharp” prismatic terminations and a “liquid-clear” interior, as these represent the species in its most perfect and high-energy form. It is a light, sophisticated mineral that offers a unique, “architectural” beauty unlike any other borate species.

Chemical Formula: Ca₂B₆O₁₁·5H₂O