Syamantaka Mani story is found in the Srimad Bhagavad Purana.
As per Hindu scriptures, Satrajit, a vassal king of Dwaraka, received the
Syamantaka Mani – a precious stone that ushers in prosperity.
Satrajit did not
share the wealth received from the precious stone with his people. So Krishna asked
Satrajit to allow him to make proper use of the precious Syamantaka Mani.
Satrajit declined.
One day, Prasena, brother of Satrajit, went hunting wearing the Syamantaka Mani. A lion killed Prasena.
Jambavan, the famous bear in the Ramayana who is also a
Chiranjeevi, saw this incident killed the lion and retrieved the Syamantaka
mani.
He then presented the precious stone to his only daughter,
Jambavati.
Now Satrajit and other people accused Krishna of
killing Prasena for the Syamantaka Mani.
Balarama, the eldest brother of Krishna, happened to hear
about the scandal. He immediately asked Krishna to find the Syamantaka
Mani and clear his name.
Krishna then set out in search of the precious stone and
tracked it to the cave of Jambavan.
However, Jambavan refused to return the precious stone. A
fight took place between the two and it continued for 21-days. Jambavan used
all his magical powers to outwit Sri Krishna but he failed in all his attempts.
Finally, Jambavan realized that Sri Krishna was none other
than Bhagavan Sri Ram. He agreed to return the Syamantaka Mani and also offered
his daughter Jambavati in marriage to Sri Krishna.