This colorful stone comes from the Milpillas Copper Mine west of the small town Cananea in the North Mexican Sonoran Desert. The mine is small and produces not very much material. As a consequence the stones achieve high prices ever since their market launch in 2006.
Depending on the mineral composition the color varies from brownish red with olive-green spots, to black areas with a metallic luster, together with blue and green areas, grown closely together like a patchwork. Using X-ray diffraction analysis (EDX) the green-blue zones could be identified as an intergrowth of brochantit (a copper sulfide) with the green copper silicate chrysocolla. The brownish to orange-red areas consist of the copper oxide cuprite grown together with another copper oxide: tenorite.
This mineral composition is typical for rocks formed in the oxidation zone of sulfidic copper-bearing orebodies through the oxidation of copper sulfides or native copper. Brochantite and chrysocolla are typical secondary minerals, often formed through infiltration of sulfurous and siliceous aqueous solutions in conjunction with hydrothermal processes.
Neither of these copper minerals is in and of itself rare or spectacular. However, their combination makes this rock a colorful collectors stone. The extraordinary color combination is best seen in stones tailored as cabochon.