pear shaped pendant featuring a swiss blue topaz gemstone
The lustre of a mineral that's described as vitreous is very similar to glass.  The word vitreous comes from "vitrum" which is Latin for glass.  Stones with a vitreous lustre are sometimes described as being "glassy" although the term is not particularly popular with gemologists because most glass is synthetic hence should not be compared to a natural mineral.  With that said, obsidian is volcanic glass which formed as a result of lava cooling so quickly that there was little or no time for crystal growth.     

Vitreous is the most common type of lustre seen in minerals and is used to describe stones which reflect light to some degree.  Most transparent and translucent gemstones are vitreous.  The term vitreous suggests the appearance or physical properties of a rock or mineral are glass-like.