CC Madhya 6.279

lohāke yāvat sparśi’ hema nāhi kare
tāvat sparśa-maṇi keha cinite nā pāre
Synonyms: 
lohāke — iron; yāvat — as long as; sparśi’ — touching; hema — gold; nāhi — not; kare — transforms; tāvat — until then; sparśa-maṇi — the touchstone; keha — someone; cinite — to recognize; — not; pāre — is able.
Translation: 
As long as it does not turn iron into gold by its touch, no one can recognize an unknown stone to be a touchstone.