Natural-colored Cu-bearing plagioclase feldspar (andesine) is mined in Inner Mongolia. The gem-quality feldspar is derived from Quaternary-Tertiary volcano-sediments related to Jurassic-Cretaceous volcanic rocks. Most of the crystals have an orangy red color, and a few have red and green zones. Natural copper-bearing andesine is an unusual gem that is prized for its color and rarity. There has been widespread suspicion that the Chinese red andesine is diffusion-treated. After much investigation of the mines in Tibet and Inner Mongolia, the authenticity of the andesine was confirmed.