Prehnite

Prehnite is a calcium aluminum phyllosilicate (sheet silicate) mineral, typically pale yellow-green, that forms in cavities within volcanic rock. Prehnite forms in cavities within basaltic volcanic rocks - the same general environment as zeolites, with which it commonly co-occurs. It crystallizes from hydrothermal fluids at relatively low temperatures (200-350 degrees Celsius) as the volcanic rock cools and groundwater circulates through it.

Prehnite has a historical distinction: it was the first mineral to be named after a person. Colonel Hendrik von Prehn brought specimens back from South Africa to Europe in 1788, and German mineralogist Abraham Gottlob Werner named it in his honor.

The most spectacular prehnite specimens come from Mali, where translucent, spherical aggregates of gem-quality prehnite form ball-like clusters that glow with an internal yellow-green light. These Malian specimens transformed prehnite's reputation from a modest collector's mineral to a sought-after gem and display specimen.

Prehnite is identified by its distinctive translucent yellow-green color, botryoidal or spherical crystal habit, and characteristic 'inner glow' when light passes through it. At hardness 6.5, it's moderately hard. The slightly waxy luster on polished surfaces is typical.

Distinguish from peridot (much more vivid green, transparent, higher specific gravity), green chalcedony (different crystal structure, typically more opaque), and jade (much denser, different luster). Prehnite's botryoidal form and gentle luminosity are its most recognizable features.