The term is "amygdale"
a gas bubble canity subsequntly filled by mineral precipitates, especially zeolites, calcite, or quartz. Often "almond shaped" (the literal meaning of the word).
They sometimes form in vertical accross layering...think bubbles in beer! Often form when lava is emplaced over wet ground. More common at the top of a flow.
All that is from my geology dictionary, I'm sure other information can be dug up.
BTW, When I was teaching intro labs, I used to put a basalt that contained calcite amygdales on the rock test, really confused some students to have a basalt with calcite phenocrysts!