touchstone

touchstone 

noun.  a basis for comparison, a standard, a reference point 

"David Bowie’s constantly morphing persona was a touchstone for performers like Madonna and Lady Gaga."

"In the continuing debate over how to stop mass killings in the United States, Australia has become a familiar touchstone. There have been no mass killings nearly 20 years."

Origin:  In the late 15th century, gold and silver was rubbed, or touched against black quartz — the touchstone — to determine the purity of the metals. This was done by looking at the color of the streaks left on the stone.