touchstone
/ˈtʌtʃstəʊn/noun
1- a piece of fine-grained dark schist or jasper formerly used for testing alloys of gold by observing the colour of the mark which they made on it试金石。
1.1
- a standard or criterion by which something is judged or recognized检验标准：
they tend to regard grammar as the touchstone of all language performance.
他们往往把语法作为所有语言运用的检验标准。