Spaced-apart elongate ribs formed on a floor of an upper surface of a coaster have sidewalls that each have a concave curved surface extending upward from the floor, and a convex curved surface extending upward from an upper end of the concave curved surface to a top of the rib. A radius of the concave curved surface is larger than a height of the rib. A lower surface of the coaster has spaced-apart nibs having surfaces formed as surfaces of rotation, each of which downwardly extends from a general lower surface of the coaster body to a bottom of the nib. A sidewall of the nib has a concave curved surface that extends downwardly from the general surface, and a convex curved surface that extends downwardly from a lower end of the concave curved surface to the bottom of the nib. A radius of the concaved curved surface of the nib sidewall is greater than a height of the nib. The avoidance of crevasses, corners and channels in the coaster makes the coaster easier to keep clean.