The answer is: An onyx

Exo 28:9 - to be part of the priestly ephod

In the story of the Exodus, the engraving of the names of the twelve children of Israel on an onyx stone adds vivid detail to the ceremonial practices of ancient Israel. Located in the divine instruction for the High Priest’s attire, this was no ordinary stone. Onyx, a gem known for its durability and beauty, symbolizes the weight and importance of the priestly office assigned to Aaron and his descendants.

The significance of the names being engraved on the stones illustrates a deeper emotional connection. Each name represented an entire tribe—think of it as wearing the identity of the whole people within the very heart of their spiritual leader. Priests served as intermediaries between God and humanity; having these names on his ephod showcased a literal and figurative pressing of the needs and concerns of the tribes before the Divine.

Interestingly, throughout various ancient civilizations, the art of engraving stones for monumental purposes was common, echoing a widely shared practice across different cultures. This thread runs deep; Israel’s forefather Abraham's name was also inscribed in the annals of future generations as a mark of faith. Moreover, onyx stones are believed to carry symbolic meanings like protection and strength, invoking a sense of divine oversight in the community’s trials and tribulations.

So, when you think of those stones flashing between the light and shadow, consider the profound connections and things wondrous it all represents—identity, community, and commitment to a spiritual path that wove together the lives of the Israelites on their journey through trials.