A lot of people talk about straightness and flatness as mathematical concepts. But I think OP means it in a technical sense, as in flat like your phone screen or straight as the edges of the screen but in nature. In this sense, flatness or straightness is defined as a finite number of measured points on a surface of which the coordinates all lie between 2 mathematicaly flat/straight parallel tolerance planes/lines. By that definition, depending on what a person would consider flat, say 0.002 mm between the planes/lines, there are definetly naturally occurring crystals that would pass that test.
A lot of people talk about straightness and flatness as mathematical concepts. But I think OP means it in a technical sense, as in flat like your phone screen or straight as the edges of the screen but in nature. In this sense, flatness or straightness is defined as a finite number of measured points on a surface of which the coordinates all lie between 2 mathematicaly flat/straight parallel tolerance planes/lines. By that definition, depending on what a person would consider flat, say 0.002 mm between the planes/lines, there are definetly naturally occurring crystals that would pass that test.