A soft vein running through a block of stone.
A filandre is a comparatively soft vein that passes through a block of stone. Because it may respond differently to tools, abrasives, moisture, and weathering than the surrounding stone, it can influence both carving strategy and the long-term soundness of a sculpture.
Etymology
French stone-working term.
References
https://www.pierres-info.fr/glossaire/index.html