Excerpt from "Chapter 2: The Road East" in Lioness Rampant by Tamora Pierce (pp. 44-46 in my copy). Coram tells Alanna the story of Miache, Zefrem and the Dominion Jewel:
"... Miache was a Carthaki waterfront thief, three hundred years ago. The Gallans hired her t'steal the Jewel from their own king, that was descended from Giamo - a great-great grandson, he was. Them that hired Miache wanted t'rule in his place.
"Miache stole the Jewel, right enough - and she kept it. She ran for the River Drell, the same that's our border with Galla and Tusaine and Maren. She might've borne it home to Carthak, too, but for Zefrem the Bear. He was a mercenary, and a good one too, headin' south on the river when he pulled Miache out of it. Before long they were lovers. She was a pretty thing, with hair like moonglow and a heart of pure ice. Zefrem cracked that heart some, though.
"When they came t'the city of Tyra, the Carthaki navy was attackin'. The local folk were starvin'. Their nobles had run; their rulin' duke was crazy. The only thing keepin' Carthak out was the walls, and they couldn't hold against Carthaki siege engines.
"Zefrem, now, he was never a man for a losin' fight, let alone one already lost. And Miache - she'd watch her own mother starve unless there was somethin' in it for her. All who knew them said it had t'be the Dominion Jewel that brought them t'stay in Tyra. They didn't even know how to use it, but it seems the Jewel used them.
"Zefrem took command, trainin' the men who were left and buildin' catapults to throw fireballs at the ships. Miache and the city's swimmers, some of them younglings, they'd swim out t'harry the Carthaki navy. They even sank some of the barges full of men and catapults. Miracles started happenin' - birds found nestin', when the city had none. Schools of fish appearin' in canals beneath the city, where no fish'd been before. Men and their families began to move into the city even durin' the war, t'make their homes and t'fight for Tyra. They didn't know why they came. It was the Jewel, callin' them.
"They saved Tyra, Miache and Zefrem. The city was a pirate's nest when they came, a sinkhole fit only for cutthroats and thieves. They made it a lawful tradin' city where a man's word was a bindin' contract. The man and woman vanished, and the Jewel came next to Norrin, but Tyra still prospers. That was three hundred years gone."
Disclaimer: the Song of the Lioness series and its characters are entirely and utterly the property of Tamora Pierce. I own nothing and after playing with the characters - for which I earn no money, mind - I always put them away neatly.