Pitch black stone that contains a trace of spiritual power.  When it comes in contact with a human body, a small trace of this power would sink into the person's body and activate their qi. The stone would absorb the qi and start to emit light. The stronger the qi of the holder, the brighter the stone will shine. After years of testing, humans already concluded its usage. They can use the brightness of the stone to determine the strength of the holder.

Referenced in the Scroll of the Way in the chapter "Ten Thousand Uniques".