Coesite is a high-pressure silica polymorph that has been found in some meteorite impactites and in mantle eclogites, both as xenoliths in kimberlite and as ultra high-pressure crustal metamorphic examples.  It is colourless in plane light and has relief higher than quartz with which it is commonly found (the quartz being a low-pressure breakdown product).   In A, two grains of high relief coesite occur with quartz rims, in a mantle eclogite xenolith that also contains clinopyroxene (substantially altered to a fine-grain turbid assemblage) and one grain of garnet.  In B, the relief of coesite is not significantly different from that of enclosing kyanite.  First order interference colours are evident in C, in a grain that is only slightly inverted to quartz.  Complete replacement of coesite by quartz commonly results in a fibrous rim with a mosaic core, as illustrated in D.  All samples are from coesite-bearing eclogite xenoliths from the Roberts Victor Diamond Mine, South Africa.  Fields of view are 3 mm across in A, 2.2 mm across in B and C and 5.5 mm across in D.  A and B ppl, C and D x-nicols.