Shikon Jewel

The Shikon Jewel, also known as the Sacred Jewel or Jewel of Four Souls, is a powerful marble-sized jewel which grants whoever possesses it with immense power. Shikon, is formed from the Japanese words, 'shi' meaning four, and 'kon' meaning soul. 四 (in Chinese writing system (ancestoral to Japanese) means four (shi). 魂 (in Chinese writing system (as well as Japanese) means soul (kon). 玉 (in Chinese and Japanese) Jade or royal stone (tama). Making the Shikon no Tama. Near the end of the series, it is revealed that the Shikon Jewel is the true antagonist, manipulating Naraku and the group into preserving its existence.

The Four Souls
The four souls described are from the shinto philosophy of Naohi (直霊): Aramitama (荒魂, Courage), Nigimitama (和魂, Friendship), Kushimitama (奇魂, Wisdom) and Sakimitama (幸魂, Love). When a person contains these four spirits at their maximum, such as Midoriko, they unite to form a really strong and powerful balance within the soul that can be used for either good or evil.

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. It was created to be held by a priest, priestess or monk with exceedingly strong and powerful spiritual powers. However, to make it unhealthy it usually makes the host demonic and or not themselves.

Creation of the Jewel
The jewel had come into existence during the Heian Period of Japan by the battle of the great priestess Midoriko and a strong and 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 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 within it still battle on. The state of this battle is influenced by the person who possesses the jewel.

History
The jewel was burned with Kikyō, but returned fifty years later through a reincarnation of Kikyō'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 if not thousands of shards (Shikon no Kakera) which scatter themselves across Japan. Inuyasha's group (which includes Miroku, Kagome, Shippō, Kirara and Sango) are attempting to collect the shards and stop Naraku from getting the jewel for himself. As of the latest episode of the anime adaption of Inuyasha, Naraku has completed a corrupted Shikon No Tama. When Naraku was killed, Kagome was absorbed into the jewel to start a new battle between good and evil.

Inuyasha followed her in and finds out that the jewel is trying to make Kagome make a wish so it can start the battle all over again. Because unlike most beliefs, Midoriko is still alive and fighting the demons within the jewel. The jewel wants Kagome to take her place and replace the demons with Naraku instead. Kagome finds out that the jewel does not really grant one's true wish. She realizes that Kikyō wished to be with Inuyasha and Naraku's wish probably wasn't an evil one as he had intended it to be. She remembers her grandfather telling her that the Shikon Jewel can only be purified by one soul that wishes to save everything. It's then that Kagome realizes the wish and makes it after Inuyasha joins her side. She wishes for the Shikon Jewel to be gone forever, thus freeing Midoriko's spirit so that she can pass on to the afterlife and sending the demons within to Hell; Naraku is shown to have awakened and finally is content by the warmness that the Shikon Jewel shined as it is destroyed, purifying his soul of evil. Three years later eighteen-year-old Kagome returns to the past in which she lives with Inuyasha by her side every day as long as she could, ending the series once and for all.