___The Little Tiger___

"The key to life and death is everywhere to be found, but if you do not find it in your own house, you will find it nowhere. Yet, it is before everyone's eyes; no one can live without it; everyone has used it. The poor usually possess more of it than the rich; children play with it in the streets. The meek and uneducated esteem it highly, but the privileged and learned often throw it away. When rejected, it lies dormant in the bowels of the earth. It is the only thing from which the Philosopher's Stone can be prepared, and without it, no noble metal can ever be created."

What is it?

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http://www.alchemylab.com/riddle_hint.htm