latter produced 60,000 carats per year, but its mining activity ceased in 2002. Diamond bearing kimberlite pipes and dykes were emplaced during the Ordovician in the Archean crust between 450–480 million years ago and again in the Tertiary. Uplifting events caused the kimberlite to be exposed. The two mines exist along narrow and discontinuous dykes around the Tan Lu fault. Porphyritic kimberlites often occur with a matrix of other materials, such as serpentinized olivine and phlogopite or biotite, and breccia fragments. The occurrence of diamonds with different materials caused a difference in diamond grade, diamond size distribution and quality. For example, the diamonds from the China Diamond Corps' 701 Changma Mine worth US$40 per carat, while the diamonds from the Wafangdian Mine worth up to US$125 per carat. See also Archean subduction Eastern Block of North China Craton Eoarchean geology Western Block of North China Craton Notes a.Ga is the short form for billion years ago; Ma is the short form for million years ago. References Geology of China Cratons Northeast China Historical tectonic plates Historical continents