a polycrystalline material
Picture a tiled floor where different workers laid patches of tiles at different angles: inside each patch the pattern is neat, but neighbouring patches meet at mismatched seams. Most metals are exactly like this — a mosaic of many small crystals, called grains, joined together, each grain a tiny crystal with its own orientation.
This structure arises during solidification. Crystals nucleate at many separate points in the cooling liquid and grow outward until they collide with their neighbours. Each becomes a grain with its own lattice orientation, and the mismatched seam where two grains meet is a grain boundary. Grains are typically a few microns to a few millimetres across — visible under a microscope once the surface is polished and lightly etched.
Because the grain orientations are random and average out, a polycrystalline metal is usually isotropic in bulk, even though each individual grain is anisotropic. Grain size is a powerful strength lever: finer grains mean more grain-boundary area blocking dislocation motion, so smaller grains give a stronger metal (the Hall-Petch relationship). Grain refinement is a favourite way to strengthen a metal without adding weight or losing much toughness.
Etch a polished piece of steel and look under a microscope: you see a patchwork of grains, each a tiny crystal at its own angle, outlined by dark grain-boundary lines. Refining these grains from coarse to fine can noticeably raise the steel's yield strength (Hall-Petch).
The grain structure of a metal, revealed by polishing and etching.
Random grain orientations make the bulk roughly isotropic, but heavy rolling can align the grains and bring anisotropy back (texture). More, smaller grains means more boundary and higher strength at room temperature.