Brainteaser #26 – A Little Series of Riddles Part. 2

As promised, here are some more devilishly hard classic riddles for you! Encased in mail that doesn’t shrink nor clink, always drinking, even when heavy they float, not sinking. Grinding on, devouring. Builds towns and mountains, within each second and minute it is found in. Small box, no hinges. Smooth surface, gold lies within it….