Although naturally occurring volcanic glass is not a mineral (it has neither a definite chemical composition nor a highly ordered atomic arrangement), it is a common phase in some rocks (especially volcanic rocks).  Lacking a crystal structure, glass is isotropic, although it may contain a variety of mineral phases, typically minerals that have crystallized from the magma before it solidified into glass.  Image A illustrates basaltic glass that is brown in colour and contains numerous small crystals of plagioclase (low relief) and clinopyroxene (moderate positive relief).  The image also contains two fairly large colourless regions.  In the lower left is an elliptical feature called a vesicle.  Vesicles are holes representing bubbles of gas that exsolved from the magma before it solidified into rock.  A large jagged region dominating the lower right area of image A is a hole in the rock (with associated cracks), probably created by accident when the thin section was being prepared.  Both the vesicle and the hole are now filled with epoxy, and all three features (glass and epoxy-filled vesicle and hole) are isotropic (and appear black viewed with crossed nicols in B).  The epoxy in the hole and cracks is strained and therefore not completely isotropic.  The rock in images C and D consists of two large crystals of plagioclase within a glassy matrix (the latter is isotropic and appears black under crossed nicols in D).  The glassy matrix consists of small shards of glass formed during a gas-rich explosive eruption, and they were flattened while still hot by compaction of the body of accumulated shards due to its own weight.  This rock is termed a “welded tuff” and most such rocks are more silicic than the basalt of images A and B.  The lighter colour of the glass in C relative to A suggests that the glass in C is more silicic (and richer in Na and Al and poorer in Fe and Mg) than the brown basaltic glass in A.  A relationship between refractive index of volcanic glass and its composition allows qualitative estimation to be made of the silica content of a glass by measuring its refractive index.  A and B (each 2.5 mm across) are from Oahu, Hawaii and C and D (each 1.2 mm across) are from White River Canyon, Nevada.  A and C ppl, B and D x-nicols.