This is not like crystalline ice, where it's a very compact ordered structure.
这种水冰不像结晶冰——结晶冰的结构非常紧凑有序。
PBS趣味科普
The ordered structures that cause iridescence in opals are called photonic crystals, a kind of crystal where the bending and reflecting of light is happening in three dimensions in a periodic or repeated pattern.
Order theory is a branch of mathematics which investigates our intuitive notion of order using binary relations. It provides a formal framework for describing statements such as "this is less than that" or "this precedes that". This article introduces the field and provides basic definitions. A list of order-theoretic terms can be found in the order theory glossary.