In the alternate reality, Spock gifted a Vulcan amulet made of vokaya, once owned by Amanda Grayson, to Nyota Uhura. Its radiation proved useful in locating Uhura (and therefore all the surviving captured members of the Enterprise's crew) when she was held captive by Krall on the planet Altamid in 2263.
Upon learning of the mineral's unique quality, Leonard McCoy opined that Spock had given his girlfriend a "tracking device", something the Vulcan never intended. Later, during Captain James T. Kirk's thirtieth birthday party at Starbase Yorktown, McCoy eyed the amulet uncomfortably, knowing its true nature. Upon Spock's pleading with his eyes for McCoy not to tell her, he let it go. (Star Trek Beyond)
Doddema thought of trinitite, a real radioactive material created by the Trinity atom bomb test that was used in jewelry before its effects were understood. Such radioactive materials would have been created on Vulcan in the nuclear wars before the Time of Awakening. Studying the Vulcan language, they settled on the name "Vokaya" as a contraction of Vokau-heya, meaning "remembrance stone" or "memory stone", as the Vulcans would use vokaya to remember how warlike they were. 
Uhura’s Vulcan amulet necklace created by the props department was designed by illustrator John Eaves. Eaves created some different designs of the necklace that had a retro Star Trek vibe. They were 3D printed and shown to Director Justin Lin for approval, then finally tried on Zoë Saldana for the fit. The metal in the definitive version was aged, while the blue stone in the middle was a synthetic opal. The props department made at least three copies of the necklace: one for Saldana, one for her double, and one in case something happened to one of the other two.