I am taking Engineering 101, and trying to get the hang of metal structure on a 'grains' level. Ok, so my understanding is this; Metals are polycrystalline materials, that can be divided up into 'grains'. These grains are basically crystals/lattices with a repeating pattern, separated by amorphous solids. The place where two grains are separated by an amorphous solid is known as a grain boundary. All correct so far?! Ok, my question is; say in a lump of Fe ore, would these amorphous solids separating the grains be made of Fe, or some impurity? Does pure metal have grain boundaries? It would seem to me they would likely form one giant crystal...but my understanding is hazy.