Crystals and their structure.
Already in antiquity it was noticed, that a common mineral called quartz can exist in the form of polyhedrons bounded by flat faces. These polyhedra constantly have the shape of a hexagonal column ended with the walls of the pyramids. Although the individual geometric forms of quartz differ in appearance, their permanent feature is the hexagonal shape of the column. The name "crystal" referred for many centuries only to such transparent quartz, which are now called rock or rock crystals. These crystals were mainly mined in the Alps, where they were cut from the cracks. The ancients thought naively, that these crystals are ice resulting from very strong supercooling and solidification of water, and the needle-like mineral inflorescences occurring sometimes inside them (chloryt, actinolite) were considered moss or grasses. Only in the 18th century. All solids in the form of polyhedrons bounded by flat faces began to be called crystals.
Forms of quartz crystals: a - correctly formed crystal, b, c, d - deformed crystals, e, f, g - crystal cross-sections.
From observations it follows, that the crystals of different substances have different forms. Quartz creates crystals in the form of a hexagonal column with inclined walls. Rock salt crystals are in the form of cubes, calcite often forms cubes-like crystals, but differing from salt crystals by the slope of the walls, crystals of other minerals have different forms. The crystal forms of many minerals are so distinctive, that minerals can be recognized at first glance.
Crystalline bodies have the ability to form crystals, characterized by an orderly internal structure, consisting in the correct arrangement of the atoms that compose them, ions or molecules. The opposite of the structure of crystalline bodies is disordered, disorderly structure of amorphous bodies. The vast majority of minerals, including gemstones, has an orderly internal structure, however, these minerals are rare in the form of properly formed crystals. That such crystals arise, special conditions are needed to enable them to grow freely, e.g.. in rock crevices or voids, where there is free space and a constant supply of substances, from which they are formed.
Structure of the crystalline body: the chlorine ions and sodium ions in the rock salt crystal are arranged correctly.
Different properties result from different internal structures of crystalline and amorphous bodies. It is known from observing crystals, that their properties differ depending on the direction. The speed of crystal growth depends on the direction, which affects the variety of their forms; have different optical properties in different directions, electric, hardness, the ability to crack along flat parallel surfaces, that is, cleavage, in inside. These properties are always the same in crystals in parallel directions, generally differ in non-parallel directions. In amorphous bodies, on the other hand, the properties do not depend on the direction and are the same at every point. Unlike crystalline bodies, which melt and solidify at a certain temperature, amorphous bodies never have a specific melting point - they soften gradually during heating and turn into a liquid with a viscosity that decreases with increasing temperature.
Crystalline bodies are the subject of crystallography research, dealing with the outer form of crystals (geometric crystallography), their internal structure (structural crystallography) and the relationships between the structure of crystals and their chemical composition and properties (crystallochemistry).
Structure of the amorphous body: The silicon and oxygen ions in the quartz glass are arranged irregularly.