The Shikon Jewel is said to be able to grant any wish; in reality, it will twist a wish to its own benefit at worst or simply not grant it at best. It's even stated in story that every single attempt to use the jewel for good has backfired. What this amounts to at the end of the story when someone is in a position to make that wish is that there's only one precise wish that can be made that will actually work (apparently the Jewel has no power to pervert that one) and anything else will result in doom. Kagome wishes for the jewel to vanish from existence forever, which is said precise wish.
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