Syamantaka (Sanskrit: श्यामन्तक), Syamantaka mani (Sanskrit: श्यामन्तक मणि) or the Syamantaka Jewel (Sanskrit: स्यमंतक मणि), is perhaps the most famous jewel in Hindu scripture, supposed to be blessed with magical powers.
The story of Syamantaka appears in the Vishnu Purana and the Bhagavata. The jewel originally belonged to the Sun god, who wore it around his neck. It was said that whichever land possessed this jewel would never encounter any calamities such as droughts, floods, earthquakes or famines, and would always be full of prosperity and plenitude. Wherever the jewel remained, it would produce for the keeper eight bhāras of gold daily. ("Four rice grains are called one guñjā; five guñjās, one paṇa; eight paṇas, one karṣa; four karṣas, one pala; and one hundred palas, one tulā. Twenty tulās make up one bhāra.") Since there are about 3,700 grains of rice in an ounce, the Syamantaka jewel was producing approximately 170 pounds of gold every day. It was also the source of the dazzling appearance of the Sun god.
A gift from the Sun God
On one occasion Satrajit, a Yadava nobleman, and a devotee of Surya, the Sun God, was walking along the sea shore, praying ardently, when the god himself appeared before him. Seeing the god in an indistinct and dazzling fiery shape, Satrájit asked him to appear in a less blinding form, so that he could see him clearly. For this, the Sun God took the Syamantaka jewel off his neck, and Satrájit saw him of a dwarfish stature, with a body like burnished copper, and with slightly reddish eyes. Having offered his adorations, the Sun God offered him a boon, and he asked for the jewel. When Satrájit returned to Dwarka with the jewel, people mistook him for the Sun God, such was his dazzling glory that even Krishna asked him to present the jewel to Ugrasena, the supreme leader of the Yadavas, but Satrájit did not comply.
Theft and recovery
Satrajit later presented it to Prasen, his brother, who was also the ruler of a Yadava province. Prasen wore it often, until once while hunting in the forest while wearing it, he was attacked by a lion, which killed him and fled with the jewel. But it couldn't get away with it, for shortly after, it was attacked by Jambavant, described as king of the 'bears' or 'gorillas' according to different scriptures, who killed it after a fierce fight and took off with the bounty. Jambavant was loyal to Rama, and was considered one of the seven immortals or Chiranjeevi.
Now there was a rumour that Lord Krishna also had an eye on the Syamantaka jewel and when the incident of Prasen's mysterious disappearance became public, the people accused Krishna of murder and theft. In order to prove his innocence, Krishna sought to find out the true culprit and recover the jewel. As he followed on the trail of the deceased Prasen, he came to the spot where the corpses of Prasen and his horse still lay, along with pieces of teeth and nails of a lion. From there he followed the footsteps of the lion, which led him to the spot of the second struggle, where the corpse of the lion was lying. From there, he followed the tracks of a bear, which finally led him to the entrance of Jambavan's cave, where the latter's children were playing with the priceless jewel. Thereafter, he engaged in furious, protracted combat with Jambavan for 28 days, and Jambavan gradually grew tired. As he was the strongest living entity at that time, he wondered who could be weakening him. It was then that Jambavan realized that he had been sparring with none other than Sri Rama himself. Jambavan, who was hot-headed but pious by nature, returned the jewel to Krishna, who later married Jambavati, Jambavan's daughter.
Krishna's marriage to Satyabhama
Meanwhile Krishna's companions, having waited twelve days for Krishna to come out of the cave, returned to Dwaraka despondent. All of Krishna's friends and family members became extremely sorrowful and began regularly worshiping Goddess Laxmi to assure the Lord's safe return. Even as they performed this worship, Krishna entered the city in the company of His new wife. He summoned Satrajit to the royal assembly and, after recounting to him the entire story of the Syamantaka jewel's recovery, gave the jewel back to him. Satrajit accepted the jewel, but with great shame and remorse. He went back to his home, and there he decided to offer Lord Krishna not only the jewel but also his daughter Satyabhama, so as to atone for the offense he had committed against the Lord's lotus feet. Sri Krishna accepted the hand of Satrajit's daughter, Satyabhama, who was endowed with all divine qualities. But he refused the jewel, returning it to King Satrajit. 
Deaths of Satrajit and Satadhanwa
After a few days, Krishna and Balarama were off to Hastinapura after there were rumours that the Pandavas have been burnt to their deaths in a fire. Kritavarma, Akrura and Satadhanwa, who had their eyes on the dazzling jewel, conspired to make use of Krishna's absence from Dwaraka as an opportunity. Satadhanva one night entered the house of Satrajit and killed him in his sleep, making off with the jewel.
A sorrowful Satyabhama rushed to Hastinapura to inform Krishna about the ghastly death of her father. Krishna and Balarama immediately started for Dwaraka to avenge Satrajit's death, hearing of which Satadhanwa fled on his horse, keeping the jewel with Akrura. He was chased down by Krishna and Balaram and finally killed by Krishna near Mithila. Later Krishna returned to Dwaraka, and upon realizing that Akrura had already fled to Kashi with the Syamantaka jewel, summoned him up, and asked him to admit his guilt. When Akrura complied, Krishna let him keep it, on the condition that it was to remain in the city of Dwaraka.
The Puranas or the Mahabharata do not say what happens to the gem after Krishna leaves the material world. There has been many attempts in identifying the true jewel and its existence in the modern Indian society.
However, some speculate that the legendary Syamantaka Mani might be in fact the famous Koh-i-Noor diamond, which was known to have been in the possession of the Mughal emperors of India, followed by the Sikh empire, and currently one of the Crown Jewels of the United Kingdom.
Whether the Syamantaka Gem is actually the Koh-i-Noor diamond is unknown. The Koh-i-Noor, of course, does not match the superlative descriptions of the Syamantaka, and considerable poetic license would have to be assumed.
Srīmad-Bhāgavatam (Bhāgavata Purāṇa)
(SB10.34.30) Lord Govinda chased the demon wherever he ran, eager to take his crest jewel. Meanwhile Lord Balarāma stayed with the women to protect them.
(SB10.34.31) The mighty Lord overtook Śaṅkhacūḍa from a great distance as if from nearby, my dear King, and then with His fist the Lord removed the wicked demon’s head, together with his crest jewel.
(SB10.34.32) Having thus killed the demon Śaṅkhacūḍa and taken away his shining jewel, Lord Kṛṣṇa gave it to His elder brother with great satisfaction as the gopīs watched.
(SB10.56.45) The Supreme Personality of Godhead told Satrājit: We do not care to take this jewel back, O King. You are the sun-god’s devotee, so let it stay in your possession. Thus We will also enjoy its benefits.
Syamantaka Ruby or Shyamantaka Blue Sapphire?
A common mistake of spelling it wrongly as "Shyamantaka" has led to the incorrect idea that Syamantaka was a sapphire (Saturn's gem). Also according to Jyotish or Planetary Gemology, Ruby is the gem of the Sun in the Navaratna.
Two verses from the Bhagavat Purana, which describe Satrajit wearing the gem given to him by Surya, the Sungod show that Syamantaka is a ruby.
- “Wearing the jewel on his neck, Satrajit entered Dwaraka. He shone as brightly as the sun itself, O King, and thus he went unrecognized because of the jewel's effulgence”. – Ref. SB 10.56.4
- “As the people looked at Satrajit from a distance, his brilliance blinded them. They presumed he was the sun-god, Surya”. – Ref. SB 10.56.5
- Cintamani Gem
- Kaustubha Gem
- Kisshoutennyo (吉祥天女)
- Kohinoor Diamond
- Vishnu Purana
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