The non-metallic element silicon (Si), with 28 wt% the second
most abundant element in Earth’s crust, served as the first inor-
ganic component of the animal skeletal system during the Prote-
rozoic (800 mya; reviewed in [1]). Elemental Si is only very rarely
found. However, in its oxidized form, as silicate/silica, it is among
the most common minerals on our planet. Si comprises almost
exclusively a tetrahedral coordination, with 4 oxygen atoms that
surround the central Si atom, and occurs as SiO2 in the thermo-
dynamically most stable crystalline form; the amorphous form of
silica is less abundant [2]. In the living world SiO2 occurs, with only
one exception, i.e. the nanorods of sponge spicules [3], as amor-
phous biosilica, in particular in plants, algae, and sponges [1].