The Jewel of the Four Souls (四魂の玉, Shikon no Tama), also known as the Sacred Jewel or Shikon Jewel, is a powerful marble-sized jewel which grants whoever possesses it with immense power. (Taken from Wiki)

This is a handmade necklace of the Tainted Shikon no Tama (aka Jewel of Four Souls) from the anime Inuyasha. The jewel is a 20mm Onyx bead. This is a great accessory for everyday wear if your a fan of the show and especially good for a Inuyasha cosplay.