Shikon no Tama

The Jewel of Four Souls (四魂の玉, Shikon no Tama) or Shikon Jewel, is a fictional, Powerful crystal in the anime and manga series InuYasha.

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 (The Shikon Jewel).

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 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 one hundred 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 in the centipede yōkai Mistress Centipede (Mukade Jōrō). 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; this is not in the 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 Centipede Mistress (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 if not thousands 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.