When a Grey Dragon is ready to serve in the army is marked with the Sacramentum (or oathstone), a carved emerald-green stone that through a ritual is embedded in their foreheads.
They say that these stones are the ones that allow gray dragons to take the lower form, but that is not the case. This is a deep bond of eternal loyalty to Mold and their homeland. Some say that this is the third eye of the arunda, since his sacred stones -and the color with which they carve his eyes in the temples- are the same.
Although the origin of the Sacramentum is not known for sure, it has been discovered that it may be related to a protective spell created by dragons in ancient times, to avoid being controlled by Deneb when they stopped serving her. It is also rumored that they could be related to Iris and the draconiats, and that it was even she who instructed the Grey Gragons to be free from the control of the Arundi of the Moon. The other dragons do not use anything similar, so this theory is usually quite accepted.
The folklore around these gems also says that this stone inflicts great pain when a dragon fails to comply with the sacred laws: don’t fear, serve, don’t lie, don’t forget, don’t break promises.
The carvings of the stones can vary among the families, being the oval and the octagonal ones the most populars. The brightness of the stones give the sensation of being like stars.