Amygdaloid, a rock containing almond-shaped cavities. The term is for the most part limited to rocks of the trap variety. The vesicular cavities in these, as in the lavas, are the result of the escape of gases, as the rocks cooled down from a melted state. Subsequently to their formation the cavities have generally become filled with some mineral, as calcareous spar, quartz, agate, chlorite, or a zeolite.