The Isis knot was protecting the mummy. These common amulets are mostly in red materials, i.e. jasper, the colour of the blood of Isis. Green and blue (other regenerative colours) and white ones, or in alternative materials are less common. Since Late Dynasty 18 any mummy was burried with one almost invariably placed on the upper torso1
Red jasper H 2.8, W 1.1, T 0.4 Undated © Musée du Louvre/Antiquités égyptiennes #E 4990
Unusual blue glaze loop soldered H 2.7, W 0.8, T 0.4 Late Dynasty 18 Private collection DA
Red jasper H 5.4, W 2.8, T 0.6 New Kingdom Dynasty 18-19 Brooklyn Museum #37.1273E
Unusual blue/black glaze H 8.6, W 4.5, T 1.1 New Kingdom-Third Intermediate Period © 2022 Musée du Louvre/Antiquités égyptiennes #E 1634
Reference
1Andrews C. Amulets of Ancient Egypt, 1994, pp. 44-45 and Figs. 49e,f