Secondary deposit of minerals found in a rounded, elongated, or almond-shaped cavity in volcanic rock. The cavities (vesicles) were created by the expansion of gas bubbles or steam within lava. Because gas bubbles tend to rise through the lava, amygdules are most common near the tops of flows. Many minerals have been found as amygdules, including some spectacular museum specimens of zeolites.

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Amygdules form when the vesicular cavities (created by expanding gas bubbles in volcanic lava) are filled with a secondary mineral such as calcite, quartz, chlorite or one of the zeolites, which are deposited by having minerals "wash" through the pores in the rock (see Porosity/Permeability). They are filled from the outside, making some amygdules concentrically layered.

"Amygdules" refer to the smaller ones. "Amygdales" refer to the larger ones and is more common in British usage.