What would give Swiss Chard a gritty texture?
The recipe you linked to has lentils in it. Most recipes with lentils tell you to rinse and drain them, and also to pick over them for stones. Lentils are seeds from a plant, and during their harvest, it's not uncommon for small stones to be picked up as well. (Same goes for rice and any type of legume.)

If you didn't check for any small stones before cooking your lentils, your encounter with "stone-like" objects is probably, in fact, due to stones.