Saphira

The star sapphire had shattered. An expanding torus of huge dagger-like pieces plummeted toward the distant floor—the shimmering shards near the walls. In the center of the chamber, hurtling downward headfirst, was Saphira. Her jaws were open and from between them erupted a great tongue of flame, bright yellow and tinged with blue. On her back was Arya: hair billowing wildly, arm uplifted, palm glowing with a nimbus of green magic.

Battle for Furthen Dur, Eragon

Far stronger than any human, Durza easily defeats Eragon, slashing open his back from shoulder to hip. At that moment, Saphira and Arya break the roof of a chamber—a sixty-foot-wide star sapphire—distracting Durza long enough for Eragon to stab him through the heart.

Brisingr, Synopsis of Eragon and Eldest, page 2

Things have been . . . unsettled . . . ever since Brom died. It has made me uneasy,acknowledged Saphira, which surprised him because she rarely seemed perturbed. They were above Tronjheim now. Eragon looked down through the opening in its peak and saw the floor of the dragonhold: Isidar Mithrim, the great star sapphire. He knew that beneath it was nothing but Tronjheim’s great central chamber. Saphira descended to the dragonhold on silent wings. She slipped over its rim and dropped to Isidar Mithrim, landing with the sharp clack of claws.

Won’t you scratch it?asked Eragon.

I think not. It’s no ordinary gem.Eragon slid off her back and slowly turned in a circle, absorbing the unusual sight. They were in a round roofless room sixty feet high and sixty feet across. The walls were lined with the dark openings of caves, which differed in size from grottoes no larger than a man to a gaping cavern larger than a house. Shiny rungs were set into the marble walls so that people could reach the highest caves. An enormous archway led out of the dragonhold.

Bless the child, argetlam, Eragon.