Syamantaka Jewel

Syamantaka mani, or the Syamantaka Jewel is perhaps the most famous jewel in Indian Mythology. It yielded daily eight loads of gold, and dispelled all fear of portents, wild beasts, fire, robbers, and famine." But though it was an inexhaustible source of good to the virtuous wearer, it was deadly to a wicked one.There was a king of the name Satrajit within the jurisdiction of Dvaraka-dhama. He was a great devotee of the sun-god, who awarded him the benediction of a jewel known as Syamantaka. Because of this Syamantaka jewel, there was a misunderstanding between King Satrajit and the Yadav dynasty. Later on the matter was settled when Satrajit voluntarily offered Krsna his daughter, Satyabhama, along with the jewel Syamantaka. Not only was Satyabhama married to Krsna on account of the Syamantaka jewel, but Jambavati, the daughter of Jambavan, was also married to Krisna. When this jewel was worn by Satrajit in a locket around his neck, he appeared exactly like an imitation sun-god. Putting on this jewel, he would enter the city of Dvaraka, and people would think that the sun-god had come into the city to see Krishna.

http://en.wikipedia.org/wiki/Syamantaka