Deities could show their powers through major manifestations and minor manifestations. The appearance of a gem (or a mineral) fell into the latter category.
These stones were made to appear by deities to inform mortals of something. This could be something as general as pleasure or displeasure, but also something concrete like the existence of an infiltrator.
The ways in which these gems could provide information were varied. For example, Selûne could make a moonstone shine for a brief moment to be seen by a follower of hers, telling them that a fellow follower was in the vicinity. Baervan Wildwanderer showed his favor by letting his followers find high-quality emeralds, or his displeasure with an emerald that was either of poor quality or even crumbled away. Berronar Truesilver could tarnish silver to issue a warning to a follower whose faith was failing.
Mortals sacrificed such gems and minerals to their deities and consecrated them.