The siliceous outer cell wall of a diatom, composed of two overlapping halves held together like a pillbox.
From Latin 'frustulum' (small piece), diminutive of 'frustum.' Adopted into scientific English in the 1800s to specifically describe diatom structure.
Frustules are incredible—each diatom species makes a unique geometric pattern from silica, and these microscopic structures have been used as test patterns for microscope lenses because of their precision!
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