Not a coin but a necklace or bracelet pendant I think. Tancremont is a place near Pepinster (in Wallonia, the French speaking part of Belgium), and while the chapel is not that old - built in 1895 - the cross sure is. A farmer found it in the 19c; it was probably hidden to protect it from being damaged or destroyed during the French revolution. The crucifix is Romanesque, maybe 11c ...
See
http://www.tancremont.be/ (Click on "Historique" to learn more (in French) about the history of the cross, and on "Galerie" to view some images.)
So the medallion is a religious souvenir from the 20c. Don't think you will get much for it, unless somebody has a specific interest and/or the piece is silver.
Christian