What is a face in mineralogy ?

Face : definition

A crystalline face is a generally planar surface limiting a crystal, which can be of various size and shape, and have streaks, secondary growths or crystallization vacancies. Each crystal face is defined by three (four for the hexagonal and rhombohedral systems) Miller indices : (hkl), h, k and l which can be positive or negative integers. The faces are denoted (111), (201), etc...

By international convention, parentheses refer to crystal faces, braces {hkl} to crystal forms (this symbolism therefore includes all existing crystal faces in a given form), and brackets [hkl] to crystallographic axes (twin axes, zones or elongation).