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A small gas bubble in igneous, especially volcanic, rock filled with secondary minerals such as zeolite, calcite, or quartz.

[Latin amygdala, almond (from its shape); see amygdala + (nod)ule.]
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Natrolite is an extremely rare zeolite at the Fanwood quarry; specimens were discovered on one occasion as jumbles of acicular crystals, no longer than 1 cm, on prehnite in a diapiric amygdule.
Gases from adjoining areas, where solidification has already penetrated more deeply, migrate towards the top half of the diapir and are ultimately frozen in place as a cluster of amygdules.
However, its position in the succession of other amygdule minerals is uncertain.
The presence of gonnardite in amygdules is problematic, however, a limited substitution of Ca for Na in natrolite was registered.
Locally, it contains mineral-filled amygdules less than 1 cm in diameter.