Years ago this black, flint-like stone would be rubbed on silver or gold to test its purity. These days, the word has been appropriated to describe a measure for determining the quality or legitimacy of something. However, it can also mean the basis or fundamental feature of something. It’s this latter meaning that is used in the example sentence.
Example: Molly had worked hard to make respect and integrity the touchstone of her design practice; therefore, when an employee’s behavior flew in the face ofphilosophy, he or she was quickly asked to leave.