Jewel of Four Souls

The Jewel of Four Souls (四魂の玉, Shikon no Tama) or Shikon Jewel, is a fictional, magical jewel in the anime and manga series InuYasha. It is sometimes named simply Sacred Jewel.

The Four Souls
The four souls described are from the Shinto philosophy of Naobi (直霊): Aramitama (荒魂, Courage), Nigimitama (和魂, Friendship), Kushimitama (奇魂, Wisdom) and Sakimitama (幸魂, Love). When all united in one person, such as Midoriko, they unite to form a powerful balance within the soul that can be used for good or evil.

Most people in the North American InuYasha fanbase prefer to use the Japanese rendering of the name, Shikon no Tama.

Form of the Shikon Jewel
Physically, it is a sphere that appears to be made of some sort of crystalline material prior to the accident that shattered it. When purified, the jewel is a soft pink to purple color, however the more tainted the soul and wishes of the person holding it, the more the color turns to a lurid black-violet. It is somewhere between a marble and a ping-pong ball in size when complete, but the shards of the Jewel of Four Souls are irregular in size and shape.

Background of the jewel
The jewel was created by the battle of the great miko (priestess) Midoriko and a powerful yōkai (demon). She was able to seize the souls of yōkai and purify them, this power coming from having a positive balance of the four souls within her heart, and in a time of many yōkai ravaging the earth those who possessed spiritual powers such as hers were considered to be as powerful as 100 samurai. Her spiritual power alone paled that of any other person alive, and it was said she could purify and destroy ten yōkai at once. Her final adversary and the one that claimed her life was created by many yōkai joined and anchored within the evil heart of a human who secretly lusted for her, similar to how Onigumo loved Kikyo (Kikyō) and created Naraku.

After seven days and seven nights of fighting, Midoriko realized that she would be unable to fight off the yōkai, and as she was drawn into the creature's massive jaws, she seized the spirit of the yōkai and bound it within her own with the last of her power, killing both of them and creating the Shikon Jewel, which burst from her chest. Within the jewel, her soul and the souls of the yōkai she bound with it still battle on. The state of this battle is influenced by the person who possesses the jewel.

The jewel was controlled by various humans and yōkai for several centuries until it ended up being rediscovered by Sango's grandfather. Before the series begins, the yōkai taijiya (demon exterminators) entrust Kikyo with the guardianship of the jewel due to her purifying powers, but it leads to her untimely demise. Naraku deceives Kikyo and InuYasha, who had fallen in love with her, into hating each other by making them believe they had betrayed one another. The purpose of his plan is to steal the jewel and corrupt it with their hatred and bitterness. InuYasha is sealed to a tree for fifty years, and Kikyo is killed by Naraku, although she believes InuYasha is responsible for her death.

A special episode in the anime series suggests this fate is due to a curse from Tsubaki that would cause Kikyo, should she ever fall in love, to lose her powers as a miko and have an unnatural/untimely death, though the actual canonity of this event is disputable as this is not present in the original manga.

The jewel was burned with Kikyo but returned fifty years later through a reincarnation of Kikyo's soul in Kagome, coming back from the future because of the jewel's power and the interference of the yōkai Mistress Centipede (Mukade Jōrō). Kagome eventually unsealed InuYasha, however, the jewel was lost to a carrion crow. While attempting to retrieve it by shooting the crow with an arrow, Kagome accidentally shatters the jewel into hundreds of shards (Shikon no Kakera) which scatter themselves across Japan. InuYasha's group (which includes Miroku, Kagome, Shippo and Sango) are attempting to collect the shards and stop Naraku from getting the jewel for himself. At the moment in the manga series, however, Naraku has gotten hold of all but the one keeping Kohaku alive. This, along with the immeasurable power and endurance he was bestowed with from consuming Mōryōmaru, makes him an imposing adversary as the series rapidly strides towards its end.

There is in fact, in the anime, controversy in the way Kikyo came to posses the Shikon no Tama. Early episodes it is stated that Tsubaki's Master chose Kikyo to be the one to purify the jewel. Later on it is said that Sango's Grandfather was infact the one who gave Kikyo the jewel so that she may purify it.

The Shikon Jewel Shards
After accidentally being shattered by Kagome, the jewel separated into many shards that became scattered throughout Japan. Many of these shards are found in demons. Despite the separation of the jewel, each individual shard grants the user great strength and is hence sought after by many demons. The manga and anime of Inuyasha revolve around the group of protagonists attempting to retrieve the shards and restore them to its original, complete form. As of volume 47, Naraku holds all but one of the shards, the last of which has been embedded in Kohaku's back in order to sustain his "undead" life (so to speak). It is anticipated that Naraku will engage in another bout with Seshomaru for the possession of the last shard, as the latter is currently in traveling with Kohaku, possibly in the knowledge of Naraku's desire of the final shard.

Fake Copies
In "The Sacred Jewel Maker" a han'yō tried to create duplicates of the Shikon Jewel. The copies, however, could not even compare to the real jewel's power, as one complete copy can only match the power of one shard of the real deal. The copy jewels were poisonous to humans and will be used up in a short amount of time. The jewel maker tried to use Kagome to forge a jewel that would make him a pure yōkai(as this jewel will be a genuine copy), but, as he saw Inuyasha's power, decided to gamble it on one last strike and swallowed a handful of jewels. He did not defeat the gang and was killed when he turned back to a human at dawn by the very jewels he made.