Quartz is extremely common in igneous, metamorphic and sedimentary rocks, and very chemically and physically resistant.  Its relief is similar to that of most compositions of plagioclase, and just slightly higher than K-feldspar.  The general similarities of relief among these minerals is evident in A, in which the quartz stands out relative to the feldspars (plagioclase and microcline), as the latter are cloudy in plane light (due to incipient alteration in this rock), whereas the unaltered quartz is clear, also apparent in C.  Quartz has no cleavage.  Maximum interference colour is first order yellow (“straw” yellow) visible in several grains in D.  Quartz deforms readily and, in some rocks, strained grains of quartz do not have uniform extinction.  Instead, sub-domains of individual grains go extinct in slightly different positions, a feature known as “undulose extinction”, seen in the central grain of quartz in B.  Quartz grains are typically anhedral in most rocks and, although quartz is uniaxial and should have parallel extinction, there is usually no hint of crystal form to use as a reference for measurement of extinction angle.  Samples are granites from unknown localities.  All views are 5.5 mm across.  A and C ppl, B and D x-nicols.