Geology
A late Jurassic granite intruded a sequence of Devonian and Carboniferous sedimentary rocks. Hydrothermal fluids from the cooling granite mineralized fractures with tungsten (wolframite, scheelite) as the primary economic targets, plus fluorite, calcite, quartz, arsenopyrite, beryl, and dozens of accessory species. The mine's commercial life depends on tungsten; specimens are an extraordinary byproduct.
The signature fluorite
Purple-and-green color zoning, often as concentric phantoms inside a single crystal. The colors come from trace yttrium-cerium combinations responding to changes in oxidation state as the parent fluid cooled. 'Porcelain blue' fluorite — opaque cubes with a milky-blue tone — is unique to Yaogangxian and one of the most-requested specimens in modern collecting.
What else comes out
Scheelite (sometimes gem-quality), wolframite (large euhedral crystals — rare anywhere else), arsenopyrite (bright metallic prisms, often twinned), beryl (aquamarine and morganite), apatite, calcite, quartz. The mineralogical diversity is itself a feature of the locality.