saying grace

I just realized that saying grace is the same thing as saying thanks. How did our word grace come to signify a kind of beauty and a (lovely) way of being? When it should mean full of thanks – grateful. Why grateful instead of graceful? And why is the other way of expressing thanks in French a cognate with our “mercy.” The words all have a kind of gentility to them — grace, thanks, mercy– but they’re not at all interchangeable.

Gentility if it were a French word would be something like humanness. Which is an idealized way of summing up mankind.