The rarest crystal structure observed in naturally occurring minerals is the hexagonal close-packed (hcp) structure. This crystal structure is less common in minerals compared to other structures like cubic or orthorhombic. While the hcp structure is found in some metals (e.g., magnesium and zinc), it is quite rare among naturally occurring minerals. One notable example of a mineral with an hcp structure is ice, specifically Ice Ih, which is the most common form of ice on Earth.
