"I have something for you," he said. He dug into his pocket and brought out something, which he pressed into her hand. It was a gray stone, slightly uneven, worn to smoothness in spots.
"Huh," said Clary, turning it over in her fingers. "You know, when most girls say they want a big rock, they don't mean, you know, literally a big rock." 
"Very amusing, my sarcastic friend. It is not a rock, precisely. All Shadowhunters have a witchlight rune-stone."
"Oh." She looked at it with renewed interest, closing her fingers around it as she'd seen Jace do in the cellar. she wasn't sure, but hse thought she could see a glint of light peeking out through her fingers. 
"It will bing you light," said Jace. "even among the darkest shadows of this world and others." -City of Bones. 

Rune-stone is set on a minimalistic silver pendant. 
Measures 1.5" (4 cm) 

CHAIN NOT INCLUDED.