Which is the only rock regularly eaten by humans?

Salt - halite, a mineral you mine and grind and put on food, which no other rock qualifies for. Everything else on a plate is or was alive.

It has been worth fighting over for most of history because bodies require it and most inland diets do not supply enough. Roman roads were built to move it, Venice's early wealth came from controlling it, salt taxes helped trigger the French Revolution, and Gandhi chose the salt monopoly for his 1930 march precisely because it was a tax on something nobody can do without.

Adding iodine to it is one of the cheapest and most effective public health measures ever undertaken. Iodine deficiency causes goitre and, in pregnancy, irreversible developmental damage, and it was endemic across inland and mountain regions worldwide - the Alps, the American Midwest, the Himalayas. Iodising salt from the 1920s onward essentially eliminated it wherever it was adopted, at a cost of a few pence per person per year, and the measurable population-level gain in cognitive development is one of the largest attributable to any single intervention.