Immobilizing or grounding an enemy champion grants 10 bonus armor and bonus magic resistance for 10 seconds, stacking up to 10 times for a total of 100 bonus resistances, and refreshing on subsequent triggers (5 second cooldown per cast instance).
This augment page for Adamant reflects public Arena Mayhem data for patch 26.19.
Adamant is currently disabled and is not offered this patch.
No grade for this augment right now: it is not live, or the source has no reliable row for it.