Planar alignment of minerals in certain metamorphic rocks, producing a layered structural fabric.
Foliation is the planar arrangement of minerals in metamorphic rocks, especially mica-rich stones such as schist and gneiss. For a sculptor, foliation matters because the rock may split, weather, carve, and support projecting forms differently along and across those planes.
Etymology
Geological term from Latin folium, 'leaf.'
References
https://www.pierres-info.fr/glossaire/index.html