Dumortierite
specimen number:
1052738
location:
Dehesa, San Diego Co., California, USA
description: The Dehesa mineral locality, situated along the Sweetwater River valley near El Cajon in San Diego County, California, is a historic open-cut quarrying site known primarily for its unique nodular formations. Geologically, the deposit is hosted within a highly weathered, localized altered gabbro complex that was subjected to intense hydrothermal alteration and low-grade metamorphism during the Mesozoic era. This specialized geological environment triggered the development of dense, radial-fibrous aggregates of dumortierite disseminated within an aluminum-rich matrix of quartz, sillimanite, and secondary sericite mica, displaying a distinctive deep blue to lilac-purple coloration. Historically, initial surface prospecting and quarrying began in the very early 1900s, with formal mining claims established around 1902 to extract the dense dumortierite rock for use as a decorative lapidary material and for industrial ceramics testing. Small-scale commercial quarrying operations were carried out intermittently through the 1910s and 1920s, but the limited spatial extent of the high-grade nodules and competing mineral sources prevented large-scale industrial expansion. Following decades of complete commercial abandonment and sporadic visiting by local collectors, the historic quarry site remains inactive and sits largely on restricted or private land through 2026. Dr. Ludwig Eger (active c. 1870-1915) was a highly respected Austrian mineral dealer based in Vienna who played a significant role in the distribution of Central European and Alpine mineral specimens during the late 19th and early 20th centuries. Operating under the firm name "Dr. L. Eger," his business was situated in the heart of Vienna, a primary hub for scientific exchange during the Austro-Hungarian Empire. Eger was noted for his professional, academic approach to the mineral trade, often supplying high-quality, systematic collections to universities and major museums throughout Europe and North America. His labels are easily recognized by their classic 19th-century typography, typically featuring "Dr. L. Eger, Wien" (the German name for Vienna) and providing detailed locality and species data in a formal, structured format. In addition to minerals, Eger dealt in paleontological and zoological specimens, as was common for the natural history establishments of the time.
Purple radiating crystals rich throughout matrix. Classic locality for the species. The specimen was originally acquired in 1912 and comes with an antique label as shown at the end of the video.
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