...even "Schlagwort" or "Stichwort" might fit in the given context. Depending on what you want to emphasize, you might also make it "Leitwort" or "Motto". If you tend to stick to the metaphoric stone, use "Stolperstein" :-)
"Nowadays, its original meaning as an ecological space itself is less emphasised and the term [biotope] is more widely known as a touchstone word
in the preservation of the environment." http://en.wikipedia.org/wiki/Biotope
"“Release” applies to some of the extra weight you’ve gained as well. Throughout the year, you can see clearly how much you hold on. “Release” is your touchstone
. It grows you throughout the year. It becomes your guiding force, not your harsh standard."
"So my word is going to be ‘Audacity’. Whenever I feel myself backing away from a challenge, an opportunity, or the chance to do something out of the ordinary, ‘audacity’ will be my touchstone word
." both: http://christinekane.com/blog/