Vesicular

Igneous rocks with a vesicular texture contain air bubbles, called vesicles. These can be round or distorted. In thin-section, vesicles are filled with the mounting medium and consequently appear clear and colourless with low relief under plain polarised light. They are isotropic. Vesicles which are infilled with precipitated minerals, such as calcite or quartz are called amygdales.

Have a look at the rhyolitic glass (L113) and the porphyritic andesite (L130) in the 1A Reference Series.