An example of a Gem Tracker.
Gem Trackers are gems that are twins, and glow when the "sister" is nearby. Narcissa Simmons is known to have at least four of those. She wears all of them in rings, on each hand. Her right hand contains a Ruby, Sapphire, and Emerald, each one the birthstone of her three children (and coincidentally, her own birthstone), and her engagment and wedding rings have them also.