Primary crystals of coesite and high sanidine (Or₉₈Ab₂) up to 3 mm in greatest dimension have been identified in a grospydite inclusion from the Roberts Victor kimberlite pipe, South Africa. Microprobe analyses show that the coesite is nearly pure silica with less than 0.05 wt.% each of Al₂O₃, FeO and Na₂O. Cell edges determined by single crystal X-ray diffraction techniques are: a = 7.143(2)Å, b = 12.383(3)Å, c = 7.143(2)Å, β = 120.00(3)°; and the space group is C2/c. The feldspar has cell dimensions: a = 8.618(3)Å, b = 13.044(3)Å, c = 7.184(3)Å, β = 116.10(3)°; and the space group is C2/m indicating a very high degree of Alsingle bondSi disorder. Coesite, previously considered diagnostic of impact processes, is reported for the first time from a natural static pressure environment. This sample provides evidence that sanidine can be a stable phase in the upper mantle and that free SiO₂ may also exist. The structural state of the feldspar indicates equilibration above 900°C and the presence of coesite indicates equilibration pressures greater than 29 kbars. The preservation of the coesite indicates that the samples never spent more than a few hours above 700°C since crossing the quartz-coesite transition.