Vokaya
Vokaya was a turquoise-colored mineral unique to  which emitted a harmless but distinctive radiation.

In the alternate reality,  gifted a Vulcan amulet made of vokaya, once owned by his Human mother , to . Its radiation proved useful in locating Uhura (and therefore all the surviving captured members of the Enterprises crew) when she was held captive by Krall on the planet Altamid in 2263.

Upon learning of the mineral's unique quality,  opined that Spock had given his girlfriend a "tracking device", something the Vulcan never intended. Later, during Captain '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. ()

Background information

This mineral was named by Daniel C. Carlson and Harry Doddema, the founders of Memory Alpha.   Beyond writer Simon Pegg reached out to them, asking their help to name the necklace's material.

Doddema thought of , a real radioactive material created by the  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. This parallels an unrealized idea for Star Trek: Enterprise, to litter the floor of Vulcan's Forge with fragments of rough green glass to reflect the idea that the Forge was formed by this same nuclear war. ( text commentary, ENT Season 4 DVD and Blu-ray)

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.

External link 
 

Alternate reality
Geology
Materials
Radiation
Vulcan language