Neptunite
specimen number:
2072153
location:
Dallas Gem Mine, San Benito Co., California, USA
description: The Dallas Gem Mine, situated in the Diablo Range of San Benito County, California, is the world's premier locality for the rare mineral neptunite. Geologically, the mine is located within a large body of serpentinite that contains unique veins of natrolite, which host the neptunite along with benitoite and joaquinite. These minerals formed under unusual low-temperature, high-pressure conditions during the late stages of hydrothermal activity associated with the New Idria serpentinite diapir. Historically, the site was discovered in 1907 by James Marshall Couch while prospecting for copper, but it soon became famous for its unique gemstone content. The mine was worked intermittently throughout the 20th century by various owners, including the Dallas Mining Company and later the Benitoite Mining Company, with the most significant specimen recovery occurring between 1907 and the early 2000s. Neptunite from this locality is celebrated for its deep black, lustrous monoclinic crystals that provide a striking visual contrast against the snow-white natrolite matrix. While the mine is currently closed to commercial production and under private ownership, it remains a legendary site for mineralogists due to the exceptional size and quality of its neptunite crystals, which are unmatched by any other deposit worldwide.
Lustrous prismatic crystals of neptunite measuring to 1.3 cm in length across the front of a white natrolite matrix. This is in good condition. Classic locality for the species. At one time this was part of the collection of Wolfgang Henkel.
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