Another Riddle

"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; everybody uses it. The poor usually possess more of it than the rich; children play with it in the streets. The uneducated esteem it highly, but the learned often throw it away.  It is a living though invisible substance that is concentrated in nature's cavities, sacred cups, and parts of the human body. It is the only thing from which the Philosopher's Stone can be prepared, and without it, no noble metal can ever be created."

See The Emerald Table