I've heard of "knock on wood" for luck or to stave off bad luck after saying something they hope remains true.
I've heard of "touchstone," a stone used to verify the quality of a gold sample, and frequently it is used in a broader sense of validating the quality of anything -- or merely standing for quality in general.
Can you add a note on the effective meaning (definition)?
In parts of United Kingdom, people touch wood for luck. Most people would use it thus,
' I will get the job, touch wood,' 'When I have passed my exams, touch wood.'
it seems to be the kind of phrase you use when you hope something will happen the way you want it to happen, and to take the bad luck off counting your chickens before they have hatched, or assuming that something will go as you wish..
I don't believe I would classify "Touch wood" as a word. It's simply a phrase that people use to mean "if all goes well" or "for luck." No harm in using it, but if you're writing fiction, you should consider limiting its use to just one character in your story.