China has 4 primary casitterite localities—1) the muscovite combinations from the the scheelite locality (Mt. Xuebaoding), 2) dark crystals from Jaingxi province, 3) the new examples from Weilasituo in Inner Mongolia, and then 4) the gem brown crystals from Yunnan.  This is an exceptionally sharp twin crystal on matrix, with rather good transparency/ translucency, and to  find one on any matrix is very difficult—they are pretty much always stand-alone crystals that have unsightly contacts from where they were broken off the host rock. 

Again, the sharpness of this twin crystal is exceptional, and it’s quite transparent by the standards of the species.