The Coming of Dragons by A. J. Lake, first book in the Darkest Age series. It was published in 2006 so that fits the time you remember reading it.
Edmund, a king’s son in disguise, and Elspeth, a sea captain’s tomboy daughter, are the only two survivors of a terrible shipwreck. They just want to go home, but fate has other plans as they are drawn into the fight against an evil warlord terrorizing their homeland. Accompanied by a mysterious minstrel and haunted by magical powers they did not seek, Edmund and Elspeth journey across a savage land of wild boars, fierce rogue knights, and black magic.
As you describe, Elspeth opens the chest and:
She turned back to the chest. Slowly, she slid the lock out of the wards and pulled them up. The lid swung open soundlessly, as though its hinges had just been oiled instead of soaked with sea. The chest exhaled a sour whiff of ancient air.
Inside, it was dark unvarnished wood, giving back no reflection from the candlelight. At first Elspeth thought it was empty, but something glimmered at the bottom. She bent in for a closer look and the thing grew brighter, as if a pale flame suddenly shone within it.
It was a gauntlet made of finely wrought silver facets, each one overlapping its neighbour so they rippled like the scales of some magical fish. Elspeth’s eyes widened. Surely no silver mined from the earth could have that living shimmer? Or perhaps some minuscule creatures from the sea’s depths, those that shine with their own light, had attached themselves to the surface of the gauntlet as the waves turned it over and over. And yet that couldn’t be, for the inside of the chest was bone dry.
Entranced, she reached down to touch it. The cry that rang round the cave froze Edmund’s blood. He flung up his arms like a shield as a white light blinded him.
But in a moment it was gone. When the light’s dazzle had cleared, Edmund saw Aagard staring open-mouthed across the cave.
Elspeth stood before them, her right arm held straight out as if it were stone. On her hand was a shimmering, silver gauntlet – and in her grasp was a sword of pure, translucent crystal.