A narrow natural discontinuity or fine fissure passing through stone.
In French stone vocabulary, a fil is a fine discontinuity or fissure crossing the stone in an arbitrary direction. A sculptor should identify such lines before committing delicate projections, because an apparently minor fil may become a fracture path during carving or handling.
Etymology
French fil, literally 'thread.'
References
https://www.pierres-info.fr/glossaire/index.html