In the Mahabharata, Jambavantha had killed a lion, who had acquired a gem called Syamantaka from Prasena after killing him.
But it couldn't get away with it, for shortly after, it was attacked by Jambavan, described as king of the 'bears' or 'gorillas' according to different scriptures, who killed it after a fierce fight and took off with the booty.
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A common mistake of spelling it wrongly as "Shyamantaka" has led to the incorrect idea that Syamantaka was a blue sapphire (Saturn's gem).