Salt is (was, at least) mined FROM the Earth. It starts in the Earth, and then you take it out so you can eat it because it's an essential nutrient. In certain times, because of it's sheer necessity especially in deserts, Salt was as valuable as Gold; but a lot more unassuming.

Be the salt of the earth → be a valuable entity pulled from rough surroundings, of far more practicality and far less obvious self-promotion than gold; nurturing of human life.