The story of the Syamantaka Gem is available in the Puranas such as the Bhagavata and the Vishnu. It shows how powerful of the curse Ganesha is.
(This story is related on the fourth day of the bright half of the lunar month, Bhadrapada, the sixth month of the Indian Calendar. Lord Krishna observed this fast to redeem himself from the stigma of telling a lie. )
In good old days, Lord Krishna built a beautiful city named Dwarka amidst the sea. There lived a man named Ugra who had two sons. Their names were Satrajit and Prasenjit (Prasena).
Briefly, Satrajit worshipped the Sun god for a long time. The god was pleased with him & gifted Syamantaka Gem. The God disappeared after gifting him the gem. Satrajit put the gem around his neck. When he entered Dwarka, everybody was surprised to see the heavenly gift. It was dazzling like sun. This gem produced gold eight times of its weight when needed. Satrajit did not part with it with king Ugrasen even when Krishna asked for it saying it would be safe with Him. Prasena, the brother of Satrajit went out hunting wearing the jewel but was killed by a lion. Jambavan (Jambavanta) of Ramayana fame killed the lion and gave it to his son to play with.