Jewels were naturally-occurring stones which could store energy. Some types of jewels were better than others at storing energy.
The jewels of Riders' swords were not actually enchanted, but the natural properties of the jewels allowed them to store energy. The colour of the jewel in the hilt matched the colour of the blade, so Brisingr contained a sapphire while Zar'roc's hilt contained a ruby. The crippled elven Rider Oromis stored energy in the yellow diamond of his sword, Naegling, in the event of a duel with a Forsworn Rider.
The sapphire in Aren (the ring of Brom Holcombsson and later Eragon) was capable of holding massive amounts of energy. It indicated, via the Liduen Kvaedhí rune known as the Yawë, that the bearer was an Elf-friend.
The Star SapphireEdit
The great Star Sapphire of the dwarves was a single massive gem located under Tronjheim. The Isidar Mithrim, as it was also known, was carved by Dûrok Ornthrond during the Age of Herran over a period of fifty-seven years. It was shattered by Arya and Saphira to provide a fatal distraction for the Shade Durza during the Battle under Farthen Dûr. Saphira later fulfilled one of her seven promises by using her semi-passive magic to repair it, leaving it with a slightly-altered, albeit possibly more beautiful, tinge.