2.50 ct PREHNITE – MALI

Prehnite

Prehnite from the Sandaré region of Mali represents the “translucent-neon” pinnacle of the silicate world. While Prehnite is common globally, I have always respected the Malian specimens for their extraordinary “electric” clarity and their unique, globular luster—a quality that has earned them the nickname “Grape Prehnite” in the trade. These stones possess a “liquid-green” or “chartreuse” hue that remains vibrant and crisp, avoiding the “milky” or “cloudy” appearance found in many other localities. For the specialist, a Mali Prehnite is a prize of phyllosilicate purity; it offers a sub-vitreous to pearly luster and a “jelly-like” depth that makes it a true “tactile” gem for the systematic vault.

The Heritage & Discovery

Historical Significance: Prehnite holds the distinction of being the first mineral to be named after an individual—Colonel Hendrik von Prehn, who brought the first specimens from South Africa to Europe in the 18th century. However, the discovery of gem-grade “bottle-green” Prehnite in Mali in the mid-1990s revolutionized the market. These specimens were often found in spectacular association with dark, “chocolate-brown” Epidote, creating a visual contrast that is now iconic in the mineral world. In fine minerals, it stands as a symbol of “organic structure,” bridging the gap between common rock-forming silicates and the elite world of “neon” translucent rarities. It serves as a reminder that the arid landscapes of West Africa still hold “aqueous” treasures of immense beauty.

Discovery: The Mali deposits are located in the Kayes Region, where Prehnite forms in the vesicles of ancient volcanic basalts. Scientifically, it is a calcium aluminum silicate. Its discovery provided mineralogists with a clear look at how low-iron, high-clarity environments can produce a green so pure it rivals Jadeite but with a superior, “water-clear” transparency. My own respect for this material grew from observing its “velvety” internal texture; the finest Malian stones possess a “radiating” fibrous structure that captures light in a soft, internal “glow” that is the definitive signature of the Sandaré locality.

Important Mines: The undisputed world capital for this elite material is the Sandaré District in the Kayes Region, Mali. This remote locality produces the legendary “spherical” aggregates that set the global benchmark for the species. For the vault, I prioritize the Malian material for its unmatched “neon-mint” saturation and its capability to produce clean, faceted gems that maintain their “electric” color even in larger carats.

Mineralogical Profile

Description: Prehnite is a calcium aluminum silicate hydroxide that crystallizes in the orthorhombic system. It sits at a 6 to 6.5 on the Mohs scale, making it a durable gemstone suitable for sophisticated jewelry. It is characterized by its vitreous to pearly luster and its unmistakable “spring-lime” to “apple-green” color palette.

One of its most identifying traits is its botryoidal (grape-like) habit and its moderate refractive index ($n \approx 1.61$ to 1.67). It has a specific gravity ($SG \approx 2.8$ to 3.0), making it feel light yet substantial in the hand. Because it forms through the hydrothermal alteration of calcium-rich minerals, it is almost always found in association with Epidote, Quartz, and Calcite. When I select a piece for the collection, I look for “even” translucency and a high “surface-sheen,” as these highlight the sophisticated, three-dimensional geometry that makes Malian Prehnite a standout for the systematic specialist. It is a light, high-vibration mineral that offers a unique, “aqueous-neon” beauty unlike any other silicate.

Chemical Formula: Ca₂Al₂Si₃O₁₀(OH)₂