A glassy texture is when the rock looks like
glass. Rocks like obsidian are formed when lava cools within minutes to
hours. This usually occurs as thin layers as the lava flows over the
land. The lava contains silica dioxide which has not had time to form
crystals. This amorphous fluid hardens without forming any true
minerals. Obsidian is actually composed of SiO2, but has no
Pumice, which has a frothy look is also considered
glassy, because if you look at it closely, the dividing walls between
the cells are actually glassy.
A pyroclastic texture is composed of
fragments, like tephra, which includes bombs and ash. These rocks
take on an irregular shapes created during volcanic eruptions. Hardened
ash flows with other pyroclastic material are grouped as "tuffs."